Though the point of the thread is who
should be in the movie, here's the likelihood for each of these guys, as based on random numbers taken directly from no particular source:
Lex Luthor - 99% (probably bringing along one of the others as a minion)
Bizarro - 60% (recognizable, could be brought in under the "failed Superman clone" pretense)
Metalo - 50% (could easily be done as a Luthor minion, but doesn't have the mainstream presence that some of the other do)
Mr. Mxyzptlik 2% (as it's been said, the CGI budget to do all his wacky things would be pricey, not to mention he's a comical imp, so basing an attempted blockbuster movie around Superman fighting a comical imp is going to be reeeeeeeally hard to get approval for)
Brainiac 75% (Probably the easiest of the group to recreate in movie form: green makeup and headgear)
Doomsday 85% (Everybody knows the behemoth that killed Superman, and he could easily be either a Luthor creation or salvaged from space by Luthor)
Darkseid 40% (Not as mainstream friendly, you'd have to include his history and Apokalips into the mix, and George Lucas might try to sue
)
Mongul - 10% (also not too mainstream friendly and could easily be replaced with Doomsday or Darkseid)