Not ‘worst actor throughout the movie’, but just when an otherwise decent actor does a really shitty job at one or two lines…
Chris Penn - Reservoir Dogs - “Why don’t you tell me what REALLY happened?”
next?
I first thought of William Shatner’s line in Wrath of Khan – “Still . . . old . . . friend.” Then I reread the OP and saw “otherwise decent actor” and realized Shatner is disqualified.
Well, hell, I was going to say Sofia Coppola’s “Daddy!” line from Godfather Part Three but she falls under the Shatner Rule as well…
Can’t remember the exact line, but when Bill Pullman in ID4 said something like “A lot of people died today that didn’t have to.” Or something like that.
Can’t remember it cuz it was so bad I refuse to watch the scene again
Easy- Vanilla Sky, Tom Cruise to bartender - “Why don’t you say it to my face…BITCH!”
I notice Samuel L. Jackson is pretty inconsistent in this regard. Can’t think of a specific line off hand, but a lot of times his lines have this quality. He nailed it in Pulp Fiction, but it seems like his range is short in some other roles.
Storm in X-Men: “You know what happens when a Toad is struck by lightning? The same thing that happens to everything else.”
You stole my line! That line makes me cringe every time, and practically ruins the entire movie for me.
“No one puts Baby in a corner!”
Ok, maybe that wasn’t the actor’ fault. I defy anyone to say that line without sounding like an idiot.
Actually, my choice would be a famous line delivered by a great actor but delivered entirely wrong- to wit: Anthony Hopkins infamous “Fava beans and a nice chianti” line from Silence of the Lambs. What ruins it for me is that lip smacking thing after the line. Hannibal Lector’s character in the books is much too dry and restrained to ever do anything like that. In the book that line is delivered so casually and drily that it almost goes unnoticed. Hopkins made the character too cartoonish and actually detracted from his intelligence and darkness with that choice. It’s always bugged me.
OK, so this is another one which doesn’t count as an otherwise-good actor - but Andie McDowell’s “Is it still raining? I hadn’t noticed” at the end of Four Weddings… is just dreadful.
Well, ignoring the “otherwise decent actor” bit, IMO it would be Tony (can’t sing, can’t dance, can’t act-the triple threat) Curtis’s “Yondah lies da castle uv my faddah” line.
If we have to follow the rule, it might be John Wayne in “The King of Kings”: “He is in-DEED. Duh sonnuv Gohd.”
Fenris
I think the actor gets something of a pass when the line is that awful.
In Purple Rain (remember that movie?) when The Kid squeels “Daddy, no!” and minces across the room in his high-heeled boots to stop his Dad from smacking the crap out of his mother.
Painful to listen to and to watch. And I love that movie.
Yeah, I had this in mind when I opened the thread.
IIRC, I read an interview with the guy who wrote the line, and it was supposed to be said casually, perhaps as she was walking away. I’d say it’s a decent line, but muffed by the director and/or Halle Barry.
It was Joss Whedon who wrote the line. The only other line he wrote in the movie was Wolverine’s “You’re a dick.” line.
Yes, he said Storm’s line was delivered horribly. There’s an interview online somewhere (I think in the Onion AV club) where he talks all about it.
Daniel-Day Lewis in Last of the Mohicans
“Someday you and I are going to have a serious disagreement”
Missed it by a mile.
Now that line, I liked.
That line was terrific. 
Yeah, that’s probably where I read it, thanks.
I think there are far worse examples than the ones posted here so far.
One I can think of off the top of my head: (I have a five year old so that’s my excuse for having seen it)
Crocodile Hunter starring Steve Irwin. In which there is a scene when his Wife exclaims “Crikey!” It made me wince. It just didn’t fit.