PS2->USB adapter is the best way for emulated stuff, because then you can pick any number of other controllers, and use ones you might already have/be used to.
there's a very slight input lag, but that pretty much only matters with things with down-to-the-millisecond timing like FRCs in GGXX and IIDX games. If that bothers you, use a parallel->psx/ps2 adapter and NTPad/PSXPad.