Only out of games I've played:

Objectively worst? Hydlide for the NES. It was so badly animated as to be impossible to make out what was going on, with a flimsy plot that was as generic as RPGs get, and had all of two tracks which were obnoxious to listen to.

The one that disappointed me, personally, the most? X-Men Destiny. Generic fanficcy plot -- rando mutant gets powers and joins the X-Men/Brotherhood. You don't get to play as the X-Men OR the Brotherhood, just...talk to them. TALK TO THEM. This ain't why I bought an X-Men game you guys, I didn't want a dating sim. And the gameplay was just a slicked-up version of the Marvel Ultimate Alliance games, except that you were by yourself instead of with a team. Everything about it was either a giant tease ("Interact with your favorite X-Men!...what, you want to PLAY them? Gee, entitled much?") or a huge letdown.

I finally sold my copy to a used games store last week -- no one else would buy it.