I have to do a hit and run because people are waiting for me, but I thought this was important enough to answer ASAP.
John's Pizzeria in Times Square (no, really): http://www.johnspizzerianyc.com/index2.htm
If you're willing to venture from the 7 a little, I'm a fan of Patsy's: http://www.patsyspizzeriany.com/index.htm (the original in harlem operate's independently. see nymag article below)
The current fad is artisinal pizza. A bunch of those places are mentioned herein: http://nymag.com/restaurants/cheapeats/2009/57893/
See also: http://www.nytimes.com/2009/07/08/dining/08pizza.html
Without clicking on the Times and NY Mag links (so stating no opinion on their reliability), I agree with Tommy's list and would add Angelo's, a mini-chain related to Patsy's, Grimaldi's, and one of the famous LI pizza joints (in New Hyde Park, maybe?). Depending on where you're staying, you may also consider Totonno's, yet another mini-chain, this one related to the burned down and (I think) recently reopened Coney Island pizza giant. I have never been to the Manhattan Totonno's pizzerias, but they are in the family so I can't imagine they would be an embarrassment to the original, which is pretty fantastic. Of these, John's is probably your best bet for convenience and reliably good pizza. The Patsy's and Angelo's spots are pretty good but a little less consistent.
one of these days I'm going to finish a tour of the old school places
-patsy's in harlem
-john's pizzeria (original location on bleecker st, not the times square satellite)
I've done grimaldi's more than a few times, as well as a few off the beaten path neighborhood joints (Sal and Carmine near [Alma Mater], and Johnny's of Mt. Vernon just north of the Bronx.) Grimaldi's is good. worth an extra hour to get in good? questionable. upshot: brooklyn ice cream factory is half a block away on the east river waterfront. with a great great view of the lower manhattan skyline. mmmmm.
eta: I ate at Veloce, one of the "new" places. Damn fine eating.
If you are interested in doing an old school pizza adventure, I would also add L&B Spumoni Gardens, Tony's on Bay, and Nunzio's, all on Staten Island, and the original Totonno's at Coney Island. But seriously, DiFara's is the best.
Little known P fact: In a past life, I worked at one of the law firms involved in the Patsy's-Grimaldi's legal battle.