If we're talking about just the emulator and not what games are involved I would say PCSX ReARMed, it's such a polished emulator and I respect the effort put into it.
But I don't have many PS1 games so don't use it much..
If we're talking about my favorite for the games I play on it.. it would be DOSBOX as all my favorite old DOS games I grew up with are there.
I want to say pcsx because of all the effort notaz put in that wonder. However, I mainly play snes era games on my pandy, so my 2 cents goes to the snes9x