Anything on Action 52 would qualify. Imagine fifty-two games on a single NES cartridge, all of which have poor graphics, brain-dead gameplay (about the typical Atari 2600 standard), and bugs aplenty. And this cartridge sells for $199 in 1991 dollars. Warioware this ain't. Additionally, the sequel to The Cheetahmen, perhaps the best-known single game on the Action 52 cartridge, is even worse than the original, perhaps due to the fact that the sequel was never completed or polished.
It's also painful to play games that had awesome potential, but ended up mediocre or worse. Sonic 2006 is the most blatant example of this, but personally the worst is Knuckles Chaotix. On the outside, it has all the indications of a great Sonic game: Colorful characters and levels, fantastic graphics, and sweet music. But play the game and you'll find that it's marred by arbitrary level design, a lack of gimmicks to define individual locations, and semi-functional badniks. I did actually like the ring-band mechanics, though.
