Tough call. I picked BSSM. For me Blood Sugar is THE album to pick for RHCP while the others have had other albums that are comparable or better than the ones listed. Oh and its also the best sounding album of the bunch, I go "WOW" everytime I listen to that album. So that kinda made my vote slightly biased.
Ten is probably the closest second although I'd vote for that as well if I could vote for 2 albums instead of 1.
Nevermind is a crucial album in music history, and so is Black album but both artists have had better albums before or after IMO.
Funny someone mentioned Out of Time as well, cos that shouldve been on the list for sure.
Oh and also worth noting is how one of the best metal albums out there missed 91 by 2 months, coming out in Feb 92. "Vulgar Display of Power " by Pantera. Wouldve made this an even tougher decision to make had it come out 2 months earlier.