Either 98SE or 95 OSR2. It all really depends on how much ram you have. 32-64 MB, then use Windows 95 OSR2, if you have more than that, then choose 98SE.
Don't choose ME for anything ever, especially not for a Mame cabinet, NT really isn't Mame suitable, and XP would cripple that machine.
Also, with that processor you should go with an older Mame version like .55, that way you can actually play most of the games (current version is way too slow for that comp).