It depends on what you mean by 'best,' honestly. Are you looking for classic post-season-two Oliver and Felicity stuff, or the darker 'what if' scenarios that twist the show's premise? Most people will point you to Archive of Our Own first, and I get why—the tagging system is phenomenal. You can filter for completed works over 100k words, which weeds out a lot of the abandoned WIPs that plague other sites.
But I have a soft spot for some older, dedicated LiveJournal communities. The quality there feels different; writers took their time with world-building, and the feedback was more substantive. A lot of those stories got migrated to AO3 eventually, tagged with things like 'Arrow Season 2 AU' or 'Slade Wilson Redemption.' Those are my personal favorites—they explore the moral grey areas the show sometimes glossed over.
Don't sleep on FanFiction.net either, even though its search is a nightmare. The sheer volume means there are hidden masterpieces buried under piles of... less masterful work. I found a fantastic multi-chapter where Oliver is trapped in the League of Assassins and Nyssa al Ghul is the central POV character, and it's only got like twelve reviews. You have to dig, but the gems are there.
My current bookmark is a crossover with 'The Blacklist' on AO3, where Oliver Queen and Raymond Reddington have to negotiate a truce between their... respective circles of influence. It shouldn't work, but the author nails the voices. That's the kind of thing I'm always hunting for—the storyline that makes you see the characters in a whole new light.