A few flash games i enjoyed for a while , they're a bit old by now, but still very good and very worthy of being given a try
Warfare 1917 and its sequel
Warfare 1944that's a nice side scrolling "strategy" games in which you must take trenches and move toward the end of the level to win the battle, then improve stuff and etc..
The NecronomiconA card game based on lovecraftian mythos , very nice atmosphere for a card game.
There's a sequel but i found it less interesting as it removed the insanity effects, making the game much more dull in my opinion, and ultimately less fun.
Spectromancer: Gathering of PowerA variant on the MTG-like card games system, lots of interesting ideas in this one, and while Richard Garfield (the guy that created MTG) was one of the designer for it, it play very differently than MTG.
Kingdom RushFun gameplay on a very nice tower defense variation
It had a sequel that while still good but it was a bit too much "more of the same" to keep me as interested as in that first one.
Sierra-7An interesting raven shield-like game, save hostages , take down tango, it's very well made considering the limitations of a flash game, the ambiance is good.
Shattered colony : The survivors Very good rts-like flash game, you're setting up your base and must secure a town filled with zombie and repel hordes coming from the outside, very replayable on many maps.
The higher difficulty is insane.