It's so hard to pick out THE worst movie I have ever seen, but I'm going with The Thin Red Line. Reason being, I paid for 2 movie tickets to see it and was too stubborn to get up and leave. It is still overhyped.
Eyes Wide Shut, There Will Be Blood, and No Country for Old Men would all make the top 5. Eyes Wide Shut is the only movie I have ever left in the middle of, though.
Usually, if it is up for several awards, I'm just not smart enough to watch it