Despite my obvious bias, I'm going to have to go with Cal-Stanford over USC-UCLA for a few reasons:
1. The UCLA/USC rivalry is primarily football-driven. You don't hear much hoopla when USC plays UCLA in say, basketball. (This might be due to the fact that UCLA and USC basketball has generally sucked over the past few years.) Only football. On the other hand, the Cal-Stanford rivalry extends to every single sport, from basketball to football to rugby to swimming.
2. The Stanford-Cal rivalry is also steeped in much more than athletics--it extends to academics as well. Not so for USC-UCLA, as neither school is viewed as the conference's academic powerhouse (and that is by no means any knock on either school).
3. The Stanford-Cal rivalry is the origin of the most famous event in college football--simply known as "The Play."
see it here:
http://alumni.berkeley.edu/KCAA_Mult..._Play_1982.asp
4. The Stanford-Cal rivalry is older than the USC-UCLA rivalry; although the OU-OSU rivalry is older than both of the others, the Stanford-Cal rivalry gets the nod because it is much more widely known nationally then OU-OSU.