The question is too open ended is the problem.
It should read more like,
Q. Which OS runs mame the fastest with a moderate amount of support and drivers?
A. 64-bit OSes. Take your pick of Vista or XP
Q. Which OS is the most supported software wise?
A. Windows XP 32-bit
Q. If Im looking to build a cheap machine to run games from the 80's, which OS should I go for?
A. Windows XP or 98, or DOS.
Q. If I want perhaps the best possible performance, but the most difficult time to set things up, with the least support available, which OS should I us?
A. 64-bit Linux
Personally I just upgraded to a 64bit processor, and while I love the performance, Im having difficulty with supporting programs, where in XP everything worked. I would say right now is a rough time to make the jump to a 64 bit OS, just from my personal experience of trying to get a front end to work. Out of frustration I may have to go with GameEx, which while a great program, is not really the best option for what I want out of my machine. However, when its the only one that works well, it becomes the best option.