Gaming Mice... what's the best one out there... on a game specific basis!

By the power of Vlad's encouragement... I'd like to start a post here that everyone can use as a guide to purchasing the right mouse for the the game. My Logitech MX518 isn't what it used to be so I set out to pick up a replacement only to be bombarded by a ridiculous selection at Microcenter. Even the sales rep couldn't really say what was best because he wasn't a gamer. Please chime in! Best mmo/fps/rpg mouse ratings anyone?