what's better?

Depends for what...

9x is simple and perfect for games and such.
ME it's simply weird...should have been skipped Nah, I guess it's ok...for most purposes...OEMs thrive on it.

NT is old, with SP6 still encountered on many corporate machines
2000 with SP2 is the best choice from all above, games work ok and you cannot compare the NT kernel with the 9x one.

Technically you can compare the NT kernel with the 9x one The NT Kernel is stable, well performing, and efficient at handling multiple tasks. The 9x kernel has strong compatibility but thats it and this is due to it allowing direct hardware access.

If you use Windows 98/ME it would be for mostly gaming, email checking and MS Word. But Windows 2000 can do this too and it would be stable. Only if you have an older PC with 128 Megs of RAM and less than 500 Mhz I would go the 9x route.

Forget Windows NT4, its dead now as is 95. In fact I would say Windows 98 is near death too, MS won't allow future DirectX releases to work on 98 anymore so it will kill 98 as a gaming platform. They did it to 95 with Direct X 8.1, I am betting Direct X 9.1 won't be 98 friendly.