I know, I thought it would be easy but then I can't think of a bad movie saved by a great performance... though I know there's several examples. The titles just escape me.
The interesting thing is the practical list might not be the most hard hitting movies. I.E. once you imagine taking that actor out of that movie, then you see how far it could have fallen down a cliff.
Adam Sandler in Happy Gilmore. Actually, if you look at Sandler in his earlier movie career, he took a lot of stupid plots, strange material, bad writing and insane premises, and made them pretty fun.
Tommy Lee Jones in The Fugitive. It was a great movie for it's time, but with age, the warts do start to show. TLJ is phenomenal.
Michael Fassbender in Prometheus. The scenes he are in versus not, it's like there are two different films going on. If Ridley Scott just stuck with android stuff, he might have gotten another Oscar out of it.
Angelina Jolie in Playing By Heart
While it's not a movie, Taraji P Henson in Empire. That was a pretty ridiculous prime time soap opera, but Henson made it into something way better than it deserved to be.