I'll hang in there until the bitter end but the ones where I came closest to walking out...
Graffiti Bridge - When we were in HS but couldn't drive yet we went to the movies almost every weekend, the only reason I didn't walk out was because it was more of a social outing for a huge group, and we didn't pay much attention to the bad movies anyway.
Nothing but Trouble - Another HS group movie so we just did our best to ignore or make fun of it.
Magnolia - I know some people love PTA, so sorry, but saw this with my GF (now wife) and neither of us liked it and she just wanted to walk but I made her stay hoping for a big finish tying everything together. Other people were actually walking out. When the frogs started raining down, she couldn't take it anymore and blurted out "What the ####?!"