I havent seen the Braniac or Robot Monster, they both look pretty bad but I would go for a film I watched at the weekend , Creature from the Haunted Sea.
Perhaps that's a little unfair, considering how long they probably had to come up with it.

Bad effects from people who were capable of much, much better work, well, Karloff in The Raven springs to mind.

And Night/Curse of the Demon? One of my all time favourites, Some people like the Demon, some don't.
I think it's excellent apart from a couple of of shots right at the end where it's so obviously a guy in a bad suit. Those 2 shots could easily have been removed without hurting the film at all.