No kidding...the best slice I've found in a year of looking on the UWS is at the Hot & Crusty Bagel Cafe on the west side of Broadway near 87th Street. I've tried it out a few times just to convince myself that it's possible.
Skip over the pastries and bagels at the front of the store and go to the back, where the pizza counter awaits. I had one of the best mushroom slices of my life there yesterday for $2.30. The crust is thin, crispy and tasty, the sauce tastes like fresh homemade crushed tomatoes, and the cheese seems to be of two varieties, with nice slices of fresh mozzarella (not grated). And lots of fresh sliced mushrooms on top. Delicious.
If this was just a pizza place and not a bagel place it would be famous. Around ten times better than Sal & Carmine's at 101st street.