Here are the stand outs as I see them:
Plants vs Zombies - Even without the MJ zombie, it still is amazing, and extremely fun.
Defender Chronicles - SOOOOO much content. There's like 200 hours of gameplay here if you enjoy grinding the high-level RPG content out, and a good 20 even if you don't. Visually gorgeous fantasy themed with tons of units at once (probably most of any TD game ever). Good sound/music as well.
GeoDefense (& swarm) - Very pretty, prefer the original because of the more unique gameplay than swarm, but TD has never looked so nice in space.
Elemental Monster TD - Mixes a lite-CCG aspect in to the game, another huge looking game.
Towers in Space - Another huge amount of content here, extremely awful VA aside, you have a very component TD game here, with 50+ types of creeps, and for the price I picked it up for (free), it's way surpassed my expectations.
Crystal Defenders - Squaresoft Final Fantasy TD, guess if you're a fan of FF, seeing the little enemies running by your screen may hold enough charm to push this up in to the consider category. Expensive though
Other not-so-worthy but still mentionable titles: 7 Cities (boat TD), Garden War (garden TD), Fieldrunners (original iPhone TD hit), Creeps (nightmares attacking your bed TD).