Nintendo Entertainment System
GPFCE is the best NES emulator available on the GP2X with great speed, compatibility and many great features including support of opening zip files and state saving that the other emulators do poorly with or simply just do not have.
Super Nintendo Entertainment System
DrPocketSNES is an Super Nintendo Entertainment System (SNES) emulator with amazing compatibility, speed and sound that includes a lot of different features like screenshots and fast forward. Other SNES emulators do not compare to this one as they are slower and much less compatible with games.
Game Boy Advance
gpSP is a Game Boy Advance (GBA) emulator with unrivaled compatibility and speed that has been ported to many other hand-held consoles. It's simply the fastest and easiest to use and has many features other emulators do not have like fast forward options and filtering to make the graphics look a lot better on a larger screen.
Game Boy and Game Boy Color
GNUBoy is a great Game Boy (GB) and Game Boy Color (GBC) emulator with many great features and excellent game compatibility. There are no other emulators for the GB and GBC consoles available for the GP2X, but it's still the best because of its features like state saving and screen shot options and would put any new contenders to shame.
Genesis, Master System and Game Gear
DrMDx is a great multiconsole emulator that can run games for Genesis, Master System and Game Gear. It's fast, compatible with most games and has a lot of unique features like save states and fast forwarding. Because of its multiconsole feature, it allows for on-the-fly running of games without the need to switch emulators.
PlayStation
PSX4GP2X is a fairly fast and compatible emulator that can run Playstation (PSX) games, a feature unavailable on most hand-held consoles. It's still in development, and with more progress, it could be able to run many more games as fast as a real PlayStation in the near future.