Second list.
National Treasure - Silly but fun, played with an earnestness by Nicholas Cage in the sort of role he was born to play. (Mentioned by someone else.)
Man on Fire - Denzel Washington elevates pretty much anything he appears in.
Payback - Mel Gibson plays a bad guy revenging himself on other bad guys.
Hook - Cheesy but lovable film filled with childish wonder. Hated by adults that have forgotten their happy thoughts.
Over the Top - 80's sound track sports movie, featuring big rigs and professional high stakes arm wrestling. Well from the bottom of Stallone's worst output, and actually probably nearer the top.
Tron Legacy - I'll second this as better than its reviews might suggest.
Some people have mentioned 'Dark Crystal' and 'Labyrinth', but both of those movies have Rotten Tomatoes critic and audience scores above 70%, which I would feel is objectively well above 'bad movie' territory. If we are going to accept "performed poorly in its initial box office", we are going to end up talking about 'It's a Wonderful Life', 'Bladerunner', 'Willy Wonka and the Chocolate Factory' (Gene Wilder), 'Hugo' and the 'Shawshank Redemption' as examples of bad movies that we like.
As for Dune, what you have there is a big budget mess of a movie that gets almost everything wrong and yet occasionally has enough of the source material peaking out from under the filthy rags it is asked to wear that it manages some gravitas amidst all the terrible direction and script adaptation decisions. Patrick Stewart and a similarly robust cast certainly helps in that regard. I personally feel that the LotR trilogy is a similar mess, elevated only by much better decisions regarding its production design and artistic direction, and the fact that the long run times allows audiences to forget just how much of the trilogy is genuinely bad. Yes, yes, they get the 'Red Day' speech spot on, but those moments where they stick to Tolkien's dialogue and staging and so get something wonderful are few and far between.