Or you could use Steel Legion figs. They all have gas masks.
In order of local popularity, I see:
Space Marines-roughly 60 to 75% of players at local tourneys use these guys. Most of us seem to own a Space Marine army, even if it isn't our first choice of force to play
Eldar-About half as many folks play Eldar as do Space Marines, the other armies. Maybe 30%?
Dark Eldar-Varies widely over time from 15% to 5% of folks at tournament games.
Imperial Guard-Thanks to the Armored Company, these guys are becoming far more popular, sometimes as much as 20% of tourney players will use IG, usually, however, it is only around 5 to 10%. I rarely see standard Codex armies though (besides mine)
Orks (mostly speed freaks)-Tied with IG actually.
Tyranids (rapidly on the increase since the codex was released)-Used to be rarer than Orks and IG, but now much more common.
Chaos-Rare to see them
Tau-Growing quickly. Should soon surpass IG in frequency. I just wish people would paint their armies as fast as they expand them
Sisters of Battle-Only seen one in action
Necrons-only seen one in action
Not very scientific, but that is the approximate frequency I have seen. Thing is, a lot of long-time gamers own multiple armies. I have a decent ork force, but I don't use them in tournament play due to the "non-GW" pieces, grossly oversized conversions, and so forth.