Time period, location, role, wealth, culture, all kinds of stuff. If you're looking for the most common, though, you need look no further than the spear. They're cheap, simple, easy to make, easy to use, easy to train with, and they work especially well for groups in formation. With a long enough spear one soldier can help out the next three or four down the line. They were basically the standard for centuries and dozens of cultures, from the Greeks to the Japanese. At least for infantry, which is all I'm really qualified to talk about. I believe Akrinor, Ekyl, and Maslab are more knowledgeable than I, but don't quote me on that.
Of course, as I mentioned, there are variations. A particularly notable example being the Romans. While they essentially copied the Greek Hoplite style of warfare in their early days, they later transitioned to shock infantry, charging into battle, throwing their pila, then stabbing with the gladius.
In general, though, the sword was more popular for self defense than warfare, somewhat akin to modern handguns.