Matthew's is a Baltimore institution, located in Highlandtown, and should not be missed, especially if you're taking in a show or an exhibit at the Creative Alliance directly across the street. In my opinion, Matthew's Pizza has the best pizza in town. I read somewhere recently noting that, although Matthew's doesn't say it, their pizza is an example of the perfect Greek pizza. Whatever it is, it's delicious.
Their large pies are big enough for one person (maybe saving a slice or two) and are thick, doughy and cooked to perfection. At dinner times, there's likely to be a wait, so try to get there early if you're pressed for a show time. Inside the place is a throwback to old school pizzerias with simple murals on the wall and formica table tops.
If it's a nice day, get carry out and take it to the nearby Patterson Park. Pies to go are simply wrapped in butcher's paper—a practice I think is quaint (and probably more sustainable).