OS/2 isnt an MS OS, it was made by IBM.
Anyway, my vote's for DOS 6.2x
If games would run under DOS these days I'd be using it all the time no doubt, it was incredibly friggin' fast, decently stable, very easy, and yet you had pretty decent control over what the system was doing.
Very nice gaming OS.
A dual boot between DOS and Linux would be pretty nice, Linux for work and DOS for games.