definitely try Porta di Roma next time. However, I think Zini's Pizza is one of the best pizza that sold by slice too.
Best Pizza by the Slice Dallas 2001 - Porta di Roma
Forget those stampeding brass bulls near City Hall: Pizza joints are the best thing downtown has going for it. Porta di Roma is the newest addition, located in a renovated storefront across from the Bank One building. We're a fan of any pizza by the slice, but this one is exceptional: wide, melting triangles of heavenly goo supported by a strong, thin, slightly crispy crust. Perfection. As a bonus--like we need one--Porta di Roma offers huge, fresh plates of pasta, cheap. We're hot on the "spaghetti olio" dish, a mound of pasta flavored with olive oil, garlic, chili pepper and a dash of Parmesan. Even comes with hot bread. Lunchtime price: $4.25.