The worst pizza I've ever had is a fight to the low end death between two NY slice joints. You can find incredible stuff at a corner spot, you can find high end shit that is incredible (not Roberta's- worth it for the atmosphere, but the pizza is no more than good), but like every-freaking-thing in NYC, you can find all range of shit- the best and the absolute worst.
BTW, those worst pizzas ever did not include the free pie you get with a drink at the Crocodile Lounge.
Speaking of which, if you make it to midtown, get a free hot dog with a drink at Rudy's.