After perusing the threads, other than the Big Two (Runequest and Call of Cthulhu) Stormbringer/Elric! seems to get the most love of all the BRP iterations, even forming the basis for other versions of the game. What makes Stormbringer special in a way that other out-of-print products, say Ringworld and Superworld, aren't?
(I mean, with the latter two your character could one-punch that giant cave troll and sent it drifting off into space!)