I don't have a specific game that I consider the worst, but there are definitely games I find boring. If a game makes me feel bored quickly, I just stop playing it, and it doesn't leave any lasting impression on me.
I Remember being an avid fan of the pokemon games, but after playing Sun and Moon I lost all interest, I can't even say it was a bad game, it was okay, but it felt as if I had outgrew the franchise. I've only recently picked the franchise again with Legends AZ and had lots of fun with how unserious it was.
I don't think I played any actual bad games yet. Maybe very buggy ones, but defnitely not bad. Though, I know that Sonic 06 is pretty bad for example lol. But, personally, I didn't like the 3DS game, Hometown Story.