while soundstage is certainly important, its not the only thing. as long as soundstage and positional audio are at a good enough level i would not concern yourself too heavily on it.
ad500x or ad700x will have the most soundstage as listed above. they are bass light though.
hd558/hd598/game one are basically the same thing. soundstage is pretty average but good. fairly neutral signature, a bit slow but good for easy listening.
i've heard the k701/k702 have a large soundstage but its also a bit unnatural and feels distant. not sure if the k7xx follows suite or not. some akg cans are not too bassy and more treb/mid focused and some can be very picky about source/amps. i've heard the k7xx is a bit easier to power, a bit more bassy and less finicky but dont have experience with them myself.
dt990 had a good sized soundstage but is not excessively large either. definitely good enough. bright treble and good bass considering its an open can.
some of the above (dt990 or akg) i would recommend a good headphone amplifier with. some (ad/hd) are fine with onboard normally or a fair soundcard will power them.
if you want deep bass, the dt770-pro80 has decent soundstage, good bass and a heavy v-shaped signature. it is closed back so will not have the same soundstage as some open cans but its more than good enough for gaming. a decent soundcard (z) can power them or an amplifier. if you went with the higher impedance models then an amplifier would be suggested.