Sometimes Maxim, but usually I carry a bank of Kayser's and a bank of Subach's. On a ship like the Myr, it's usually a powerful weapon on the first bank (2p) and my gen-purpose on the second (4p). Same on the Ares, though reversed. It seems that the AI reacts too well to shield-breakers. By the time I whittled down their shields enough to do real damage, I don't have enough time to kill them. On the other hand, with quad-fired or oct-fired Prometheus or even Subach, it's only a few hits until the shields and the hull are both down or out. Plus, my favorite trick is, when the PromS is available, is to start firing and then swap the second cannon in, for 30% faster firing.
Not many shield quadrants will survive 8 PromS bolts at a time.