I don't much care for the half life 2 but no wear near the worst game I've ever played, nor are any of the games anyone has listed here, well outside of DarkNight_DS's general statement of really bad 2600 games and what not
anyway really bad games, SSSnake and encounter at L-5 for the 2600 are way up on the list
Bebe's Kids on the SNES is really really bad as well
everything on the RCA studio 2 is complete garbage, but the worst game I've ever played would have to be Club Drive for the atari jaguar
the music is terribly annoying,
the visuals are probably the worst professional 3d game ever made, take a first gen 2600 and turn it in to cubes instead of squares on you've got the visuals, then sprinkle in slow down from and already awful 10 frames a second
gameplay wise it is a complete mess, it controls like a poorly greased, 3 wheeled shopping cart, and has complete absurd physics, when you crash into a wall; instead of stopping you simply drive up the wall until you hit an invisible barrier
and the game modes are basically all aimless driving sequences