I've come to especially enjoy TDs where I have more to do than simply build, such as the delightful Toy Soldiers which I picked up for my shiny new Xbox. There's less building and mazing than most TDs but being able to hop into a plane or tank, become a sniper or simply control any tower is brilliant and it can really play into the game. I can rely upon my dogfighting abilities to save money by building fewer anti-air guns, or I could handle infantry myself in a tank, letting me build more anti-armour turrets. Pleasant!
Revenge of the Titans is another great tower defense if you fancy more than simply building, charging you with manually reloading turrets and collecting mined resources, plus there's an interesting tech tree too. A more conventional alternative is Sol Survivor, which mostly plays like a standard TD but gives you control of an orbital weapons platform too so you can rain 'splosions down upon the creeps.
Oh, also, watch for all the QuakeCon news. And happy Friday the thirteenth.