BPB is extremely hard at first, but if you learn how to play it and polish your short-game, it becomes rather easy: BPB F9 and B9 are almost as easy as STA F9 and KIA B9 for me. Although I shot a 30 on STA B9 recently, I find STA B9 and KIA F9 to be the hardest set of nine holes in the three RG courses... much harder than the front and back of BPB.
I haven't played any of the courses other than STA, KIA, BPB, and Oakmont, but I'd say out of those choices, Oakmont is the hardest course in general.