Probably AddictingGames.com. I honestly rarely play flash games, but when I do it's usually from there.
When I was younger I would always play on Miniclip.com, now I usually play on Addictinggames.com, both great sites. I would say Miniclip looks "cleaner" but Addictinggames has better games and a wider varity.
I don't play flash games that often anymore but when I did, I'd play on www.newgrounds.com great site and plenty of fun game and flash videos to watch. I've played on others but that's my main one.
I just google them and play on sites as I go.
LoL I don't play flash games anymore... But when I was playing, the most was on miniclip.com and y8.com
I used to play Flash based games on both Miniclip and addicting games, although I found that Addicting had a wider variety of games which I enjoyed playing.
Flash games are a last resort for me. Usually it goes like this...
Wanting to waste 5-10min : miniclip
Wanting to waste 30min+: dragongamez (lame name I know), they usually have my favorite kinds of flash games, the mystery/I'm trapped and need to escape out of this room ones.
And active players too!