I've had all of those species at one time or another, albino socolofi was by far the least aggressive, in fact way too mellow to hang on his own among other mbuna (but as Coler mentioned the blue socolofi was by far the most aggressive, had to be taken out, because he took over half the tank and the whole pile of rocks along with its terrotories).
I've voted for the afra, even though it's a little more aggressive than an albino socolofi, it both can stand his own with other mbuna and it's mellow enough to even go into a peacock tank. My adult afra along with a lab is in the peacock tank right now and I see absolutely 0 aggression from him.