First, this is a great question, and plumbs some of the very worst that DC has to offer ! Of course the worst DC film ever is a title that's contested by Green Lantern, Catwoman and Jonah Hex, but these two are serious contenders.
Hmmm.... I think that Superman IV was a fantastic premise, but executed incredibly poorly. The one saving grace of the film is Christopher Reeve, who is still charming and likeable as Superman, while surrounded by total **** - I mean, even Gene Hackman is pretty awful in that film. The special effects look like a couple of film students put them together.
Batman and Robin.........well I'm trying to think of any redeeming features, but it's pretty awful. Clooney is the most lacklustre Batman ever (he makes Adam West look pretty formidable) and the supporting cast are resoundingly dull (except maybe Alfred) and working with a pathetic attempt at a story.
Add to that Schumacher's cartoony visuals, and Schwarzenegger's terrible puns.
To be honest I struggle to decide which is worse. Neither of them is a guilty pleasure, really more of a nightmare that I try hard to forget.
I suppose if someone strapped me to a chair and made me watch one, I would find Batman and Robin slightly less painful (but not by much). I remember watching it in the cinema, and waiting for it to get good.....and it never did .