1. WinUAE - do I need to say anything about this one?
2. ZSNes - new version is great with multy-save state etc.
3. Spectaculator - great emu with many options and one downfall - you have to pay for license
4. DOSBox - almost perfect, but missing some important stuff as save state
5. pSX - nice PS emulator that runs much better then ePSX
The other that didn't make the list but are great emulators as well: PJ64, Gens, FreezeSMS, Visual Boy Avance, MAME, MESS, WinApe, WinVice...