The most important factor when choosing a gaming mouse is comfort. No matter how good the mouse is, how many cool features it has; if it's not comfy for your hand size, grip style and play style, then you'll not like it. Mouse weight also plays a small role here.
Second most important factor is reliability, especially reliable tracking. Nothing will annoy you more than a mouse that has tracking issues, no matter how comfortable it is for you.
First look at the above two before you start looking at features, eye candy etc.
Now, my 2c on a few "mice" I've owned over the years:
RAT 7/9 series:
The most tweak-able mouse w.r.t comfort of course

However, I just couldn't get a proper setup on mine; it just feels too wide.
Solid tracking on all surfaces
Great features (# buttons, software, etc)
Excellent build quality.
A tad too heavy maybe.
"Lift Off Distance" is low which is great.
Logitech G500:
Quite comfortable, not too big.
Solid tracking.
Lots of buttons.
"Lift Off Distance" is very high which is not so great (depends on your playing style), it didn't really bug me.
Very small amount of "native/sensor" positive acceleration, but not really an issue.
Has a cable issue (wires in the cable where it enters the mouse break. Google it)
Best mouse I've owned (for my preferences); lasted 2 years before the cable broke (which I later fixed).
Logitech G9X:
Designed to be a claw grip.
Same positive qualities as the G900 (rock solid tracking).
Very small amount of "native/sensor" positive acceleration, but not really an issue.
"Lift Off Distance" is the same as the G500: high
I'm just as comfortable with the G9X and I was with the G500 even when it's more geared towards claw grips. You get two casing that fit over the mouse body that provides you with some size options; the bigger casing is quite big.
Roccat Kone+
Feels great, looks great, has a ton of features.
Unfortunately I cannot recommend it: Buttons failing after a month of use and had serious tracking issues. Maybe I received a dud.
Razer DeathAdder 3.5G
(Bought a broken one at Esquire's blue tent sale this weekend and fixed it up lol)
Feels and behaves a lot like the G500.
Great build quality
Solid tracking
"Lift Off Distance" is high, feels about the same as the Logitech(s)
Razer Mamba 4G
Damn excellent mouse; perfect in so many ways, but:
Take note that the the "Lift Off" distance is extremely low and although the software provides you with the ability to adjust the distance however it hardly makes any difference.
Hope this helps you, as you can see I've paid a lot of school fees regarding gaming "mice"