I've never heard of Christopher Reeve's performance being under-rated. While I agree Brando's performance is totally phoned-in, I must say I do enjoy Hackman's Luthor. Maybe not the best interpretation of the character, but - at least in the first movie - he has a great energy level, and is certainly fun to watch. Not so much in II, and I haven't bothered with the other movies yet.

Superman Returns was a one-time viewing at best, with good highlights from Spacey (who you could tell he had fun with the roll.)