Windows XP Pro on Bochs.
This was on Pegasos II G4 1GHz, and it was way too slow to be usable.
On classic, I'd say Win 3.x is the best option speedwise. I remember trying Win95 on A1200 060 and it was quite unbearably slow.
[EDIT]Oops, missed the dos card bit. Depends on the card I guess, the fastest 486 should run win98 I suppose. It won't fly, however.[/EDIT]
