Any movie with Tom Cruise and Nicole Kidman in it. I don't mind either of them on thier own, but together they are the pits. I especially hated Eyes Wide Shut, I walked out after 20 minutes.

Quoted from Killakoala

Romeo and Juliet, the DiCaprio version. To date, this is the only movie i have ever walked out on. I asked for and got my money back too. I vowed never to see another DiCaprio movie again and i have kept that promise to myself

I loved that movie, as I like most of Shakespeare's  plays/movies and especially different interpretation's of them.
Wonder if you have stuck to your 'vow'!!! You will miss a good movie called  Blood Diamond.

The worst movie ive seen is 'Gigli' starring Jennifer Lopez and Ben Affleck. It had like no plot and the script was crap.

It received 6 Razzie awards in 2003 for Worst Picture, Worst Actor, Worst Actress, Worst Director, Worst Screenplay, and Worst Screen Couple. And a year later, the film won a seventh Razzie for "Worst Comedy of our first 25 Years...

Escape From Hell - 100 minutes of mysoginistic exploitation crap about women in prison

Bronx Executioner - poorly edited with footage from another Italian film spliced in. Not to mention the audio dubbing

Vanilla Sky. That movie was DREADFULL.

well, there's quite a range of 'worst' movies i can think of. actually the list of really good movies is comparatively small. there's an alternative film and music festival launched by a film director from former yugoslavia, emir kusturica, best known as the fellini from the balkans. the special concept of this festival is btw. bringing together important film directors like e.g. jim jarmush, nikita michailkov, etc. etc. with young still unknown talents. a wide range of s.c. author's films is shown and discussed at this festival in a place where professional communication and exchange is still possible. kusturica built a little village in the mountains for this purpose. the village is inspired by the production design from one of his films called "life is a miracle'.

the very first edition of this film and music festival was opened with a surprising ritual which symolises the device of the festival which is advancing real cinematographic art.  
have a look & have fun (english subs):

Hot Tub Time Machine.