I am not sure what qualifies as good pizza but the best pizza slices according to me are on the "Patriarch Evtimii" boulevard across НДК. You just start from НДК and walk ahead until you cross the street. There you cannot miss them because there are always at least ten people waiting in line, that's how good they are ;) However I'm a huge fan of the surprises, especially corn, and not sure if it will suit your taste. And yeah, I've driven across town just to get some ;)
As for regular pizzas I prefer "О'Шипка"'s 'cause they contain a looot of yellow cheese, however some regard them as too fatty.
As it seems "the best pizza" is quite a hard thing to define ;)