I would read through this thread:
It is where all the Tower Defense people live on QT3 : )
I would agree with @robc04 that Defense Grid 1 would be a great starting point; the original Plants and Zombies game as well. Dungeon Warfare was popular this year with many on the forum too.
Orcs Must Die 1 & 2 were alot of fun combining TD with some first person shooting. In particular Orcs Must Die 2 was fun to play in co-op. I would stay away from the latest Orc Must Die Unchained which is their lastest version and unfortunately was made FTP and does not appear to be well received.
I am currently working through Gemcraft Chasing Shadows and I would personally suggest that it not be the one to start with, as it can be quite complicated.