Gosh, that is... A very difficult question to answer for me. I play terrible games for the fun of it (often regretting it in the process).

There's a lot of games I could choose from: Shark Attack, which is a generic SHMUP game with an awful soundtrack (despite allegedly being made by a musician); Freedom Planet, a game that has an actually good sountrack and graphics, but suffers from an identity crisis that makes it very unfun to play; If fangames count, I could argue that Super Mario Super Quest is one of the worst games I've ever played, having the worst physics in a game ever and also having more sound effects than a poorly made Mario Maker stage... At least in Mario Maker, Mario doesn't make a sound every time he grabs a coin... @~@;


Out of all the games I would choose, though... I'd say the worst game I have ever played is Mordecai and Rigby in 8-Bit Land.

Now, here's the deal: I expected the game to be alright. The developers that made Hey Ice King! Why'd You Steal Our Garbage?!!, which is a legitimately good licensed game and I do recommend trying it out, also worked on this Regular Show game, so I was hoping for a decent game.

Instead, Mordecai and Rigby in 8-Bit Land is one of the worst licensed games I've ever played. I didn't even get to the 2nd world of the game before I quit playing it.
To begin with, the game has terrible collision detection; until I saw a review of the game, I had no idea that you were supposed to be able to jump on enemies. The game has powerups, yes, but they're all extremely confusing and terribly implemented; the mullet (based on an episode where one of the main characters, Rigby, gets a mullet so he can essentially do a Falcon Punch) is your only reliable method of attacking, since it shoots a projectile.

I managed to get to the 2nd world of the game the first time I played it, which was also the only time I managed to beat the boss. The 2nd world turns the game into a space shooter, and rather than let you learn how to play like this, the game throws you right into an incredibly long stage with no checkpoint as far as I remember. I also experienced a huge glitch here where I died and got health at the same time, killing me but still letting me move around the level... I couldn't beat it since I could no longer interact with anything afterwards, sadly.

So I quit on the game and attempted to play it again last year. It was as bad as I remembered and now I can't even beat the first boss (the game didn't save my progress to world 2).