I actually sat through all of Ishtar. Kept thinking that with that talent, it had to get better. I think my brain is still bleeding.
Second runner up is that Robin Williams disaster "What Dreams May Come". It was touted as a Robin Williams movie, which, at that time, meant hilarity. No clue in the trailer about the real plot. Opens with one of his kids getting hit by a car in front of their house with the entire family watching. Then everybody else dies after much agony. The single most depressing movie I have ever seen.