CSPSP, JPCSP, and ppsspp_win are your three main choices. All have their pros and cons, and since PSP emulation isn't exactly top-notch right now, it's worth using all three because some games might work in just some emulators.
Also I think the emulators still required decrypted binaries for the ISOs, meaning you'll need your PSP handy to do the decryption (just needs to be done once per game), unless they've updated to add support for it.