It depends on what type of games you're playing!
If you play MMO's like SWTOR, WoW, etc... then the best bet is a mouse like the Razer Naga (or any mouse that has the buttons on the side). It adds a lot of functionality to your mouse, and enables you to bind a lot more spells.
Another thing to look at is the feel of the mouse, which can be difficult when buying online. I like to put a mouse in my hand and see how it fits my hand before I buy it, because everyone has different hands.
It's all preference really, don't just buy a mouse because someone else says it's the best; buy one because you think it's the best.