The Pit is the biggest, but it's far from being the best fanfiction archive, in any fandom.
The fics aren't read over by moderators before being accepted, which bothers me. Many people would be very willing to work for FFN for free, if just for bragging rights.
But anyway.
Unless I'm linked to something on the Pit, I don't usually visit there. My main fandom is Harry Potter, and the HP section of FFN's been overrun by Mary Sues, OOC fic, chatspeak, etc. The LotR section's quite similar...
I prefer the SugarQuill for HP fanfic, and occasionally Fiction Alley, though I don't consider that as good (and besides, while FA's open to everything (though every story's read through before being accepted, so there is some quality control, even if there's still a gazillion 'Sues), SQ has very strict rules for the fic that's submitted.), but I'm biased, because the SQ is a strictly R/Hr fanfic archive, and I'm a dedicated and kind of obsessed R/Hr shipper.
I do use the Pit for
my own writing, though. My drabbles don't fit the guidelines of the SugarQuill. I've also submitted one of my fics to FA...
Ah, damn, I'm rambling again. God, I ramble a lot. So I must shut up now.