Easy choice for me.
MS1 - Simple but still stands up!
MSx - MS2 But a bit better (think about it they use emulation so the slowdown of MS2 would be in the emulated version and that's a no no!)
MS5 - I always liked that one
Whatever happens please, PLEASE don't release MS4 next Total garbage realease 1, 2 and X as art of a package and then head straight to 5. Having said that 6 looked cool..
What I think will happen though is that they'll release MS4, complain about the lack of sales and then stop......