If you're hunting for that specific Olicity arrowverse content, AO3 is basically your only realistic option these days. The tagging system there is so precise you can filter for 'Oliver Queen/Felicity Smoak,' then add things like 'Post-Season 3' or 'Canon Divergence' to get exactly the right flavor. I tend to sort by kudos or bookmarks to find the community-vetted highlights.
Finding the 'best' is subjective, obviously. Some of the most kudo'd works are epic slow-burns that rebuild the whole show, while others are these perfect little missing moment one-shots that slot right into an episode. My personal favorites are the ones that explore Felicity's tech skills as more than a plot device, giving her real agency. The '5 times' and '10 times' trope fics are also a solid entry point if you want quick reads that nail their dynamic.
Honestly, the quality on AO3 can be shockingly high—better prose than some published novels I've read. But you have to wade through a lot to find it. I'd avoid the 'Complete Works' filter initially and look at works-in-progress with consistent update schedules; sometimes the hottest ongoing fics haven't accumulated the stats yet but are absolutely brilliant.
FanFiction.net still has an archive, but it's a ghost town for new content and the search is terrible. Tumblr has snippets and headcanons, but for full, polished narratives, AO3 is the hub. I'd start with authors like 'somewheressword' or 'tdgal1'—their back catalogs are deep and they really understand both characters.