Yes I know “bad” is subjective but
Madonna in Evita
Kevin Costner in Thirteen Days
Hayden Christensen in Shattered Glass* and Life as a House
Justin Timberlake in Social Network
Stallone in… damn there must have been something!
*In fact I am pretty sure that the only reason he did not get nominated for awards for this was due to Star Wars backlash
The very first thing that came to mind, although I actually thought he wasn’t good at all in Life as a House.
Eh, I think Costner has given more than his fair share of good performances. He doesn’t have a lot of range, but when he finds something within his wheelhouse, he can be very good. See Bull Durham, Tin Cup, A Perfect World, Open Range, and The Upside of Anger.
Maybe not a great performance, but Sam Worthington was decent in Terminator: Salvation, whereas he’s been abysmal in everything else I’ve ever seen him in (Avatar, Clash of the Titans, Last Night, Texas Killing Fields, Man on a Ledge).
Rocky, of course it was written by him for him.
Oscar, Demolition Man and as the voice of Weaver in Antz. He actually does comedy surprisingly well, at times.
Unless, you meant Frank, in which case, I got nothin’. 
Nicolas Cage in “Leaving Las Vegas.” Only performance of his I can stand.
Keanu Reeves in the first Matrix movie. I think Keanu has a lot of charisma, & I enjoy watching him, but I’m the first to admit he’s not a good actor.
Keanu Reeves-Parenthood, The Matrix. Clip from Parenthood that proves he can act. Spoilered for slightly NSFW language.
I thought Adam Sandler could have earned an Oscar nod for his role in Punch-Drunk Love.
They weren’t good in those movies.
Rocky and maybe CopLand.
Will Ferrell-Stranger than Fiction
Andie MacDowell in sex, lies and videotape - always comes to mind as the example of a part that seems to perfectly fit a very limited range.
Marky Mark in Boogie Nights.
Paris Hilton is a hoot in Repo, the Genetic Opera.
I thought Keanu Reeves did pretty well in Little Buddha. He was playing Siddhartha, as parents tell the stories to their children, so his speech patterns did not seem out of place.
I think it was called Prisoner of Second Avenue. Excellent performance due to the fact that he was on screen for all of two minutes and got the crap beat out of him by Jack Lemmon.
Reeves was quite good in I Love You to Death, Tune in Tomorrow, Feeling Minnesota, My Own Private Idaho and Little Buddha. His best roles were usually in obscure movies which fit his talents and which most people haven’t actually seen.
Dorothy Comingore is clearly a terrible actress, but it works very well in Citizen Kane.
Daryl Hannah in the Kill Bill movies.
It seems that there haven’t been too many compliments about Bill Paxton on SDMBCS, but he was good in One False Move (1992). IMHO, he has some other good, maybe not great performances.
Costner’s was possibly the worst accent ever caught on film.
I offer Tom Arnold in True Lies.
People think Bill Paxton is a bad actor? I mean, he’s not Daniel Day-Lewis or anything, but he’s perfectly decent:
One False Move
Apollo 13
A Simple Plan
Tombstone
Frailty
And of course…
Aliens
[QUOTE=runner pat]
Keanu Reeves-Parenthood
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Beaten to it. He brings such depth to an otherwise very cookie-cutter character, I don’t think any other actor could have done that better. But then, I’ve never seen him as a bad actor, just as a wrongly typecast one. He really belongs in the quirky little character parts.
It’s such a wonderful movie, though I can’t stand Steve Martin otherwise. Really great acting from nearly everyone involved, and all the bit-players get to shine.