First up: Vector TD
Tower defence games have exploded in popularity in recreant years, which makes sense, given that they can be produced relatively cheaply by small, independent teams, and platforms such as the Minis store allow those indies to publish the game fairly cheaply as well.
The Minis store has its fair share of tower defense games, but for my mind one of the best of them is Vector TD. It's nothing pretentious to look at, but it is still stylish, the music suits the visuals perfectly to create a 'Tron' atmosphere, and the gameplay itself is rock solid.
It's not an innovative game or anything, but the power balance of this various towers is pretty much spot on, allowing you plenty os strategic options, the difficulty level is high but rewarding, and there's nothing fiddley about the controls. Simply put, Vector TD is a well made game, and for less than $10 on the Minis store, you can't really go wrong.