Hi anon! I’ve discovered fanfics very late, mostly because English isn’t my language and I basically learn how to speak it on here, so there was no way I could understand what I would have been reading 3 or 4 years ago. So I’m an AO3 child, so to speak.
I rarely use Gossamer and I find AO3 a lot more practical and modern. I don’t know which one is the number one, I feel like people are mostly posting on AO3 now, but if you’re looking for emblematic fanfic and old ones, they’re surely on Gossamer.
I’m also probably not the best to advise you. I wouldn’t say I read A LOT of fanfics. When I see some on my dash here, and I know I love the writer, I do, but I always run out of time. There’s probably a hundreds fics I’ve planned on reading, and still haven’t!