Probably not the worst I ever read, but whenever a question such as this comes up my mind immediately goes to one of the Tarot books by Piers Anthony. I don't remember which one, it was just in a pile of books people left in a dorm one time and I had nothing to read. I finished it, but I can't tell you anything about it other than the vague recollection that I hated it.

a novelization of one of my favorite video games

I suffered through it because I love the franchise so much and it wasn't that long but holy shit, I was the writing quality of a grade schooler but with added unnecessary and gross romance between two children.

Profiles in Courage - John F. Kennedy

Should have stuck to being a president… maybe it’d land differently now, but in like 9th grade, it was a total slog.

The Historian by Elizabeth Kostova

Timequake. I love Vonnegut but I just remember it being impossible to follow and overall not interesting.

On the Beach by Nevil Schute

There was a movie adaptation, which was also shit.

Worst book I've finished?

Probably Fellowship of the Ring.

Return of the King was great, though.

There are worse books I started and put down.