Off the top of my head, a few of the worst acting efforts (to use the word loosely) I've ever seen:

-Peter Spitzer in The Meateater (1978) - A legendary bad performance. Here's a good summation of his thesping skills: www.brainsonfilm.com/meateater.html

-Christa Lang in Sam Fuller's Dead Pigeon on Beethoven Street (1972) - This girl's more wooden than a pallet of two-by-fours.

-Sofia Coppola in Godfather III (1990) - Nepotism at its most disastrous. Not only couldn't she act to save her life, but she was ass-ugly too.

Andie McDowell, Four Weddings and a Funeral.

Denise Richards in "The World is not enough". As a rocket scientist.

As Rolling Stone put it: "You can fake a lot of things on screen. But you can't fake intelligence"

Heather Matarazzo. Period.

Sly Stallone in Driven

Rolling Stone is (as usual) wrong. Of course you can fake intelligence (You mean Russel Crow isn't a Nobel Prize winning genius?). The problem isn't that they cast a beautiful woman as a Nuclear scientist, the problem is they cast a beautiful woman who can't act as a nuclear scientist. Ingrid Bergman played scientists and doctors because she was a good actor not because she was a brilliant scientist. I'm tired of hearing people point to Denise Richads casting as unbelievable because of the part she was given to play, she would have been equally unbelievable if she played a hairdresser or a hooker. She sucks.

Alan Rickman - "Robin Hood:Prince of Thieves"

Where did that Spinal Tap refugee with the wildly teased hair come from?

Ben Affleck - Pearl Harbor

Now that is just mean - he was one of the only good parts of the movie.

Now if you would have said Kevin Costner in a lot of his films, including Robin Hood, I might have agreed w/ you. Oh well, everyone's tastes are different and that is what makes this world so enjoyable.

I completely agree w/ Denise Richards in her 007 appearance.

I nominate Freddie Prinze, Jr. and Matthew Lillard to be the two actors who are consistently terrible in any film they get near.

Jake Lloyd, Phantom Menace

While I agree 100% with FPJ, you need to see Mathew Lillard in SLC Punk before you discredit him entirely. The guy actually can act. He just chooses roles that don't allow him to prove himself.

Keanu Reeves...well, many if not most of his roles but particularly Bram Stoker's Dracula.

SLC Punk=Great movie!

Nancy Kelly in The Bad Seed. The worst job I have ever seen in any film.

Honk!

Jeremy Irons in Dungeons & Dragons.

Joely Richardson as Aunt Charlotte in The Patriot

Andie MacDowell in anything.

Come on, Andie MacDowell is not that bad. She's good in Groundhog Day and Green Card.

Hayden Christensen in AOTC.

Beyonce Knowles in Goldmember.

I agree about Denise Richards in THE WORLD IS NOT ENOUGH.

Hal Sparks in any QAF episode. Just awful. Of course so are the scripts and the dialog, so it may not be entirely his fault.

The entire cast of BATMAN AND ROBIN.

I really like her in green card. She played well off of gerard depardieu.

Kevin Costner in that movie, what was called, oh yes...everything he's been in.

I gotta disagree with this one. Everybody (including myself) assumed she was going to suck because she doesn't have previous acting experience, but she nailed the role (especially if you are familiar with the characters she was parodying). Better than Liz Hurley, and a LOT better than Heather Graham.

Speaking of Heather Graham, my nomination is Heather Graham in anything (except maybe Swingers). She's a Stepford wife.

I won't hear of it! He was perfectly cast in The Big Chill.

Everybody in American Psycho 2.Pathetic!