I'm not counting games I couldn't load or that wouldn't start (this mainly happened in the last years of the C64 when I'm pretty sure a lot of software companies had resorted to using low-quality cassettes). To be honest, a few of these would be better off going under "biggest disappointments" rather than "worst games".

Dizzy Panic (let's ring every last drop out of a franchise)
Sonic Spinball (ditto)
Ratchet & Clank - Size Matters (yeah, and so do decent cameras and gameplay that doesn't rely on glitches and unfair deaths for its difficulty)
Chuckie Egg 2 (somehow they missed the point of the original)
Turbo the Tortoise (according to the review this was the game that beat Sonic and Mario. Er, yuh. Some reviewer was obviously mixing the Valium and smarties)
Bride of Frankenstein/Frankenstein Jnr (wander about for about twenty seconds, get killed by a ghost you can't do anything about)