I enjoyed Warcraft III's millions of custom tower defenses an immense amount and Sol Survivor is the first commercial TD I've played that learns from the archetypes. You're offered a choice of 'executive officers' with their own towers and support powers--active, targeted abilities from lowly orbital lasers to nukes--so it's not simply "plonk towers down; watch." I look forward to trying the co-op and competitive multiplayer.
Shatter is also fun. I haven't played that as much. Fun though.
Hey, it's Saint Patrick's Day! Time for degenerate American frauds to pretend they're Irish, get ratted and yell "WHOOOOO YEAH BRO!" You sicken me.