Sanctum 1 with all the DLC maps and towers.
Edit:
I'm quite a big fan of any game that allows me to build "mazes of death" and then watch the little aliens walk through my path of doom. Sanctum 1 is one of the few games that allow me to do just that.
With Sanctum 2, the maps are smaller and you don't have a lot of freedom with tower building. It is more focussed on the FPS part of the Tower Defence / FPS hybrid. It is also more designed for multiplayer whereas the first one can be played properly in SP and MP mode. I know they've done a lot of updates on Sanctum 2 to remove some of the restrictions and I don't know what else, but I haven't really taken another look at it since then.

