I was seriously thinking of making a thread about this just last night. I think we should make a list and the OP should have links to all the best games.
My vote is Gem Craft.
http://www.kongregate.com/games/gameinabottle/gemcraft
Your progress through multiple levels, and if you die, you dont lose. You just go back to the main screen and pick another level.
It saves your progress so you can revisit the website and pick it up again. The game has an assload of levels and you usually unlock two for every one you beat.
Instead of permanent awards you get experience points and levels. At any time you can visit your skills screen and put points into special abilities, or wipe them out and build them up again at no cost.
In the battles you dont have lives. Every time an enemy makes it through you lose a little bit of mana. If you have plenty of mana saved up it wont matter too much if a few get through, and it doesnt subtract from your score.
Once you get really powerful you can revisit old levels to try for a higher score (and more experience points). Supposedly if you max out your score on all the levels you can unlock secret areas.
Last but least: Its one of the prettiest tower defense games.
2nd Place: Vector.
http://www.candystand.com/play.do?id=18047&CID=mochi
This one is cool because...... I dunno, but its fun. Good music.
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