There are two that stick in my mind as not worth the effort.
The Core: If they had paid experts in all of the related fields of science to come up with inaccurate and implausible science for a movie, they couldn't have done worse than what actually happened. I couldn't find a single thing about it that wasn't a character cliche, scientific BSing or ham-handed acting and directing. I can usually find something, anything about a blockbuster that's a little bit redeeming, but not here.
Jacob's Ladder: If this had been a Cannes short, it would have been stellar. The premise worked but the whole thing dragged. On. So. Sooooooooooooo. Long. It was painful to watch the characters interact, and in the end, it just died because it was so slow, dull, and pointless.