hong said:A 3.5 pit fiend is a CR 20 monster. By definition, it is supposed to be a reasonable encounter for a 20th level party.
It's not supposed to be a "reasonable" encounter (What's a "reasonable" encounter against one of the rulers of hell, anyway?) - it's supposed to use up 25% of the party's resources.
Which might highlight what a miserable failure the CR system is, but really has no bearing on which monster is more interesting / can do more stuff in combat - the 4E or the 3.5 Pit Fiend.