13. Pixeljunk Monsters
OMG how I love the pixeljunk games... Pixeljunk Monsters is 13 on my list for many reasons. For starters, I'm not really a fan of the tower defense genre yet I absolutely adore Monsters. Why? Because unlike other Tower Defense games, PJM is pretty innovative and inventive. You don't just wait for waves of enemies to come... the player is constantly going from tower to tower dancing for them in order to upgrade, searching through trees to find coins, and looking for gems to upgrade and buy towers. In short, you're actually a character and there is NEVER down time. It's kind of similar to Plants vs. zombies in that respect.
Also, the art direction and music are fantastic. And the amount of polish and content is unprecedented compared to other tower games. Pixeljunk Monsters also has like this super Mario bros overworld where you can unlock other areas and replay levels to get more shit to further unlock other areas! And there's also a bunch of challenges players can do to unlock ps3 trophies. (and yes, they are quite challenging)
I would argue that Pixeljunk Monsters Delux on the psp is the best version because it has 2 person multiplayer, more towers, and a new world with a bunch of levels to play (plus it's only $10!). Fantastic game, will always hold it dear to my heart