The emulator which I use the most... Epsxe 1.2.0 w/ the Pete's combo specials! (CDR,SPU,GPU). Next is Adripsx Dos/Win, and to say the least, I'm impressed with the DOS version - quite fast even without HW acceleration.
If sony was smart they would release their own PSX emulator, that way people who knew computer could use it and people who didn't know computer could buy a PSone. But by the time that happens ePSXe will rule the world, and it will be as big as Microsoft! (thats how long it would take sony to realize the money making potential)