A lot of the most innovative T Ds
are, sadly, still 'just' mods for Warcraft 3, rather than actual separate game products. Burbenog is excellent, but only if you have friends or don't mind waiting on pickup groups - it is NOT a game you can solo. It's basically elemental Rock Paper Scissors, only with more elements, and a good overlapping composition is required to succeed.
For other mods, Element TD and Skibi are still great, too. Skibi is the only one of the three that utilizes mazing. Element is solo, Skibi is sort of almost soloable but not quite.
For a 'real game' that hasn't been mentioned yet, I'd recommend Go Home Dinosaurs. It has some unique tower placement mechanics, and although it starts out easy and looks childish, a lot of the later levels are surprisingly challenging. You only build a small amount of towers, but your towers are highly distinguished from each other, and the levels end up feeling like jigsaw puzzles where you try to cram pieces in till they fit 'just right.'
I tried to get into Dungeon Defenders, but the mechanics felt clunky and obtuse, and I really got sick of seeing all that DLC. Orcs Must Die is basically what I'd consider an essentially perfect take of the same fusion genre as DD.
Furthermore, I think Guantanamo must be destroyed.