Onsluaght is by far the best of the flash games, you have to put up a strategy on where to put them and make combos and such.
You should really try it out.
Also, agreed TD games are the best flash games by far.
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS