I'll vote for Sara's, a kosher restaurant in the Miami area with pizza and many other fantastic dishes. I believe that their frozen pizza is available in kosher grocery stores in other parts of the US.
Thank you all for your ideas -- I love to travel, and am a big pizza fan. If I'm in San Jose or Chicago, I'll have to try your suggestions. (Or maybe I'll splurge and have a pizza shipped from Lou Malnati's.) If I'm in Milan ... I'd imagine that every pizza is delicious!