...would be to go through the pairing tag on AO3 (archiveofourown.org). There tends to be pretty good fic there, and a lot of good Marvel fic, too (at least, from what I've seen). As with most archives, there are some bad fics (and even badfics!) there, too, but the majority tends to range from good to great to fantastic.

But yeah. Go to their website, and type the pairing you're looking for into the search bar. After that...you can filter by kudos or hits if you like (or date or author's name and all that). Alternatively, you can just choose a fandom and do the same (if you're not looking for a specific pairing). That generally works.

~DF