has to be. GTA V.
Depends on how you look at it, I suppose. Example, GTA is a parent's nightmare. Bubsy 3D had awful controls, which ruined the experience for gamers.
However, I'm going to say Mother is the worst game I played, the random battle system was pretty dang awful and it was kind of hard to avoid battles. In some cases, you could get out of one battle only to go into another one a second later. At least Pokemon had repel to help reduce random encounters and Earthbound showed enemies on the map.
Sonic 2K6 and Bubsy 3D are worthy mentions as runners up.
Edit: I did consider Mighty Morphin Power Rangers on Sega CD, but I really don't consider it to be a game as much as it is an interactive movie.