A Holy Grail War, a la Fate franchise, of course! If someone could add more to my idea, it's very welcome. I am not very sure where this is going either. Oh, and Fate/Stay Night spoilers abound.

Darth Sidious and Lord Voldemort are both capable Casters, as their attacks are based on magic. But as Lord Voldemort doesn't have the capability to employ melee weapons, Darth Sidious are to fall under another class, perhaps as Saber. Fits with him fighting with a sword.

What measures the power of Heroic Spirits, should both have the right to stay in the Throne of Heroes, are fame, as defined by their popularity amongst the people in the world, and Mystery, defined by how mysterious their skill, magic, or weapons are.

Darth Sidious is rather talented when employing the Force, and also fights very well with his lightsaber. Having high charisma allows him to employ politics to his gain, allowing to control the Republic for decades, then rule as Emperor. His unfulfilled wish is to... well I have no idea, but it will be important in determining his actions. His Fame stretches many planetary systems, and lasted for millennia, seeing that we have a movie in his dishonour. His Mystery also is great, since we humans could not grasp the concept of The Force and manipulate it to our will.

Voldemort has split his soul to eight, seven stored in vessels, and one remains in his body. He inspires fear and have managed to escape death, both physically and metaphorically by inspiring fear. However he is defeated by a hero, and his unfulfilled wish is simple vengeance and total control of the world of wizardry. His Fame is short, but not to be underestimated, despite knowledge of him and his actions are censored to the Muggles by the Minstry's efforts. His Mystery is also less, as his magic is understood by many and his campaign is known by all magicians.

Darth Sidious may be dismayed if he knows the Fuyuki Holy Grail is tainted will Angra Mainyu. Why would he want to destroy? Siths want to rule, and wish machines that interprets all wishes destructively is useless to them. But Voldemort will find it useful, since he will just ask that all who are not pureblood magicians and mages killed. Straightforward enough.

I believe Darth Sidious and Lord Voldemort will have equal footing in this arena. Darth Sidious' ability with his sword will massively increase, but so does Lord Voldemort's magecraft. It's up to their masters and their skills to carry the day.

I say canon thrown aside, as the oldest hero is Gilgamesh, no Heroic Spirits has been chosen since early 19th century, only Counter Guardians, and a host of other things that run counter to the three canons.

Thus, this is my half-cent.

I can't imagine Voldy and Palpy have just been dropped into an arena together with an unbreakable compulsion to fight each other, so how do they even meet?

Someone, somewhere, referred to the most recent Star Trek movie as Star Wars: Into Darkness, claiming it was set on the planet Urth in the Star Wars galaxy. Ha ha, funny review, but let's take that and run with it. The Harry Potter series actually takes place on a planet in the Unknown Regions, shortly after the rise of the Galactic Empire.

An Imperial scout ship happens upon the planet, and by total fluke lands in England, right next to Voldemort's hideout. This is Voldemort at the height of his regained power, I guess - somewhere in Book Six? I don't know, I never read that far.

So what would Voldemort do about the human-form high-tech alien who's just landed near him? Um... short out his shuttle due to ambient magical fields, probably! Then I guess probably kill him on general principles.

But what's this? Another total fluke, and Palpatine was paying special attention to that scout (it was an envoy to contact Mitth'raw'nuruodo of the Chiss Ascendancy, since you insist on knowing), and noticed it had gone missing, and where. He sends down a probe droid, which also fails - but not before capturing long-distance footage of the grounded shuttle and Voldemort's base.

Now, this anti-technology field is something Palpatine can get behind. Imagine building a giant space station to project it onto a planet - a sort of 'Stone Age Cannon' if you will (Sidious' note to self: have to think of more awesome name). The inhabitants would be either rendered defenceless, or adequately punished for a minor transgression, at much less expense than dropping a fleet of Star Destroyers on them. So Palpy sets a high-altitude watch over Urth, and specifically over England, to see how Voldemort is making it.

The observations reveal that Voldemort is clearly employing some kind of Force technique, albeit one not mentioned in Sith records; he is also manifestly a Dark Side user, which makes him both a potential ally and a potential threat. Hmm.

Palpatine is regrettably unable to use Vader in this endeavor (technology, you know), so he instead slips one of his Hands onto Urth. Let's make it Mara Jade. She's tasked with getting close enough to Voldemort to figure out which category he falls into: ally or threat.

How does she do? That depends on a few questions I don't have the answer to:

1/ Are there magical 'wards' and alarm spells in Harry Potter? If so, Mara can't spot them (since they aren't actually based on the Force - that's just Sidious' interpretation), and gets caught when she tries to sneak in. Since she's never been noted for the strength of her Force abilities, unless she can sneak back out again she probably gets AK'd.

2/ If she succeeds in sneaking in, Mara will need to draw a conclusion about Voldemort's stability. How likely would he be to accept an alliance? Palpatine has previously 'allied' with indigenous Force users (such as the Nightsisters), and ensured they never leave their planet. But... Voldemort's never struck me as the most stable type. I think Mara would classify him as 'just wants to watch the world burn'. Please correct me if I'm wrong.

In the event that Mara either dies, or tells Sidious an alliance is impossible, Sidious probably decides to wipe the slate clean. Is there a spell to counter orbital bombardment? Mmm... probably not. Sidious wins.

If Mara proposes an alliance, she'll probably be in charge of negotiating with Voldemort (since she's already on the ground, has the defensive skills to keep her safe, and has the added help of possibly reading his mind). That alliance will consist of the Empire sending non-technological supplies to Voldemort (armour? Spice? Money?), while Voldemort sends someone up to the orbiting ship to teach Palpatine about their form of 'magic'.

No, says Sidious, I wouldn't dream of asking you to come yourself - you're much too busy in your exceptionally worthwhile war against Dumblewarts.

Which may lead to a temporary victory for Voldemort, if he sees through Palpatine's plan to keep him on Urth forever and sends, for instance, Bellatrix Lestrange. If she can act sane for long enough to reach Sidious' presence, she could probably hit him with an AK before he can react, killing him. It's a suicide mission, but it should work.

But that is a temporary victory. Let's be generous and assume that Palpy's death causes the destruction of his ship (he's a load-bearing Sith Lord), so there's no immediate retribution. But... Palpatine has clones waiting to take his spirit. Unless Voldemort can get away from Urth before the Emperor comes back, he and his entire planet are toast.

(And could he? That depends on the precise effect of magic on technology. I suppose with some delicate use of the Imperius Curse Voldy could claim an Imperial supply shuttle and its crew... but why would he bother? He doesn't know that Palpatine can come back.)

In the end, Darth Sidious wins. He either nukes Voldemort from orbit, locks him into an alliance which never lets him get off Urth, or dies, comes back, and then nukes Voldemort from orbit.

Sith Lords don't fight fair. And they don't fight hand-to-hand if they can possibly help it.

hS

IT'S BEGGING TO BE WRITTEN.

And Tom's not above murdering people in their beds.

So yeah, we'll go with Voldemort for that one.

This (Some language issues) and Of course the latter one is Vader and Gandalf, but still.

I personally see Sidious as winning. While Voldemort would have no compunction about killing, Sidious has shown he is perfectly capable of destroying entire planets. So on killer instinct, I have to give Sidious.

Now close combat. The only evidence of Voldemort fighting in close quarters that I recall is his duel against Dumbledore in OtP. Two individuals known for spell casting going up in close combat. Sidious in Star Wars Canon was shown taking down three members of the Jedi Council in very short order. He then stalemated (or held back depending on your view) Mace Windu. The second if not best swordsman in the Jedi Order. Then we see Sidious effectively defeating Yoda (the best or second best swordsman and strongest force user in the order). So close combat ability goes to Sidious without question.

Ranged abilities. Both have lethal abilities at range, and both have been shown effectively demonstrating both of them. So I will give this one a wash. Even for both.

X-Factors:

Allied Forces: Voldemort has his Death Eaters, all are very significant, nasty individuals quite willing to kill for their master. But Sidious has at a minimum the entire Clone Army (and subsequent Imperial Military) at his disposal. Ultimately I think Sidious could just through enough soldiers at the Death Eaters to eventually overcome them. Further Sidious has his apprentices, Darth Maul, Count Dooku, Darth Vader (depending when in Sidious' rise to power this takes place). That also does not even include things like The Emperor's Hand. Sidious also has his elite Imperial Guards with Echani Training (insane fighting ability). I have to give this to Sidious as well.

Machinations: The simple issue here is Voldemort did take power in the UK. And he was able to work in the shadows for quite some time, remaining unnoticed. But at the time everyone thought he was dead, and he remained in hiding. Sidious managed to conquer the Galaxy. His true nature was never revealed until the very end. And he managed to maneuver himself into absolute power, while being an incredibly public figure. The Jedi, who were looking for him, never found out until it was too late. Sidious gets the win easy here.

Immortality: Neither is a true immortality. Voldemort might have an early edge with the Horcruxes, but if it came down to it, Sidious could have either his apprentice or entire army hunt them down and destroy them. Voldemort would need to find the clone bodies, destroy them, and fight whatever defenses were in place. He does not have the numbers that Sidious does, Sidious would be able to destroy them quicker. Edge Sidious.

So Final Count:

Killer Instinct: Edge Sidious
Close Combat: Edge Sidious
Ranged Combat: Even
Allied Forces: Edge Sidious
Machinations: Edge Sidious
Immortality: Edge Sidious

Ultimate Answer: Darth Sidious wins.

First off, does each combatant have their methods of immortality intact? If so, Voldemort wins the first time, and then Sidious has a better chance each time he comes back in a clone body (though his sanity will dwindle). I think it really hinges on whether the Force can be blocked by magic, or spells can be blocked by a lightsaber or the Force. Otherwise, the first AK or Lightning/Choke to hit decides the battle, assuming Voldemort avoids getting close enough to be dismembered by a lightsaber.

Star Wars Legends canon, he conjured a Force Storm capable of destroy an entire space fleet, corrupted Luke Skywalker (temporarily) on their second encounter, and many other things.

Normal canon? Fought 3-on-1 against 3 Jedi Masters, killed 2 of them and stalemated the third, Mace Windu.

Fought one-on-one Master Yoda, and won.

Why are those important? That means he had superhuman speed and reflexes, telekinesis, electrokinesis (shoots lightning!), Force sense (more or less like the Spider Sense) and had a blade made of concentrated plasma.

Was a master strategist, always one step ahead of the Jedi Order, he's the one who masterminded the Clone Wars, and benefited from both sides, not to mention that he was the Supreme Commander... of both sides!

Created Darth Vader. Nuff' said.