While I'm one of those people who doesn't believe there's a "best" anything, I can't believe no one has yet mentioned Speer 135 gr. +P Gold Dots. While they were specifically developed for maximum performance in snubbies, it apparently performs just as well in longer barrels. According to some sources, the GDHP's performance has been on a par with the longtime favorite .38 Special FBI load (158 gr +P SWCHP).
For a standard pressure .38 Special, the Federal 125 gr. Nyclad HP (AKA "Chief's Special") has a pretty good reputation.
But, again, I don't think these loads or any other load would be the best under all circumstances. There are too many variables involved in self defense situations for anything to be 100% all the time. Shoot the load that you and your gun shoots the best.