I wouldn't say it's the best, but I'm a big fan of Beggars and grew up on their pizza.
DiGornio's was an awful choice from the beginning. I can honestly say I rarely bought pizza at the park during their stint. Last year, while at a Cubs game I actually commented that the setup they had with D'Agostino's was something I'd like the Sox to have with a local place. Really glad the Sox went this route (even though the setup will be slightly different as Beggars will be essentially sending frozen - but partially cooked - pizzas to the park per my understanding).