Threads like these are best reserved for the Mass What Should I Get thread. That being said, Kingdom Rush and Kingdom Rush Frontiers are my all time favorites. Fieldrunners 2 is pretty great. Go Home Dinosaurs is on sale and is pretty good, as well.
Try searching general discussion for threads with tower defense or TD in the titles. You'll find like 30 or more of this thread already Here's one from just last week so the info will be more recent: http://forums.toucharcade.com/showthread.php?t=195147
