Sennheiser HD700, extremely comfortable, pretty easy to drive, excellent for FPS. It's open back, but that can be good especially if you get a mod-mic, as you'll hear your own voice when you speak (with closed headphones there is a tendency to shout when you can't hear your own voice).
If you were spending a bit over $800 or scored a good used deal, HD800, but you may need a more dedicated amp. The HD700 is very very close to the HD800s soundstage.
I play a fair bit of FPS games, CS:Go, Day of Infamy, BF1, will be getting Rising Storm 2 when it comes out. Another headphone that I really really like is the Sennheiser HD250. It has a signature that is almost like an HD650, but with a more precise high-end. As a closed headphone it really creates a realistic soundstage. Overall I feel the HD700 is a better choice for gaming though. It is vivid, and will give you the edge in FPS.