Tough question, since there's a lot of good stuff on AG. I will therefore name my faves for several types of game I personally enjoy a lot:
RPG: Monsters Den - Book of Dread
Old-school dungeon RPG without the annoying navigation in first-person perspective. Especially the new chapter is cool, as you can face the boss whenever YOU feel like it.
Tower Defence: Creeper World Training Sim
It may be a commercial 'trick' since it is just the training sim, the music alone makes it worth the play. The game can prove to be quite challenging at times, but is never impossible. Fair warning, though: there is no updating. If you like updating, play GemCraft 0. You will have plenty of updating possibilities...
Miscellaneous: Phage Wars 2
I sincerely regret having rated it poorly because I liked the first game better. It turned out the second one was nicer in the long-run because it offers you a way to create your own personal strategy. It may be all about numbers, I liked it anyway. This one is a must-play if you like taking over enemy 'bases' based on sheer numbers.
But I suggest you work your way through ALL the games on AG. There are lots of cool games there!