I'd say it depends on how much you like to have extensive button options on your mouse, so you can split your movements vs. abilities accordingly! I personally rock Razer's Naga Trinity for MMOs, which has easily swappable plates on the side for 2 buttons, 6 buttons, or 12 buttons. Typically I leave the 12 button option on so I can get a full hotbar on one hand :smiley: just my preference though!
My regular ol' work mouse is a Logitech though, since it's nice and light - so I'd say decide what matters to you in terms of features, weight, shape, etc., and go from there! There are definitely a lot of excellent options out there, but it really all comes down to your own personal preference <3