There are better looking and fancier games but GeoDefense and GeoDefense Swarm are brilliant. They are very hard in parts but awesome. I always go back to these two. They are iPhone games but look pretty good on the newer iPads that use their retina graphics. Still looks ok on iPad 2 as well according to my wife.
Radiant Defense is also out for iOS? All that guy's games are brilliant, there are some very clever TD concepts in Radiant Defense. I assumed it was android only since I have never seen it mentioned here.