The Fancy Pants Adventure series .
This is a classic.
Riddle School
This series is really fun and humorous.
Harry Stickmin
My favorite group of stick man games.
Super Smash Flash 2
This game is amazing, way better than the first one. It's pretty much what the title hints, a flash version of Smash Bros. (with a slightly different roster). It has been going through demos which lead up to the final game. Currently, you can play against CPU or local Co-op, but there'll be online features soon.
Anything made by jmtb02.
Seedling, a game based on LoZ, is also really great and makes you think. Nitrome has a unique style of flash games. Super Mario Crossover, Portal: The flash version, all great games. I probably forgot to mention some really amazing ones.
But, this is my favorite.
