Since I can't actually think of the worst game ever, I'll pick one at random: Borderlands. The loot lottery was already old when Diablo II rolled around, and aside from a few flashes of humour, most of Borderlands was a vapid, directionless mess that revolved almost entirely around the completionist drive to finish as many ♥♥♥♥ty fetch quests as possible. Good artwork and music but that's about it. The fundamental gameplay was just so monotonous that any high points are just about entirely invalidated.
Rock n' roll adventures - PS2. You play as Elvis Presley in a boring music-world. There is no plot, the courses was supereasy (so easy so I wouldn't even recommend it for a 3 year old). And there was only 3 courses... It wasn't a demo.
I finnished it within 20 minutes. And now I can't even give it away.
The Sims 3 - DS. This is the worst Sims-game I've ever played. You can't do anything, not even decide what your sim is going to look like. The entire game is all about working and do "opportunity-missions". That's it.
I traded it for the Sims 3 pets for 3DS, and that was a huge difference.
Chronos Twin (or Chrono Twins?) I can't even remember the name.
Basically it was sorta like the old MegaMan games, platformer-shooters. The gimmick here was, being on the DS, a time travel story. Essentially, the top screen was the present and the bottom screen was the past. Or something.
You'd control both screens with one control. So if you needed to jump over a projectile on the bottom screen, you could jump INTO a projectile on the top screen. This created a veritable clusert**** of a game, as it was like playing two stages of MegaMan at once. Sometimes on one screen there would be an abyss, but platforms on the other screen, so you could jump onto said platform on that screen and wind up floating in the air on the other.
The story (what I read of it) wasn't terribly good, and the translation was terrible. I don't know if it was intentional or not but it seemed to be a direct translation, with no re-working the sentences.
You could get hit three times, then you'd have to restart the level.
Granted, some people love it, but I couldn't stand it. The first game I ever traded in.