Niña's 2 Restaurant & Pizzeria (located at 486 Metropolitan Avenue) was always a decent place for slices, but sadly I went there today and they were closed. Their quality had up and down points, but they were pretty much the best place in the hood for slices. I also enjoyed ordering pasta from them too. I also noticed the trucking rental place behind their location was has moved, but I don't think it's related as there was a "For Rent" sign at Nina's.
Nina's had a special place in my heart because it was a bit of a landmark for me when I moved into Williamsburg back in late 2004. The service was always sweet and very Italian, which can be more rare than you might think for a pizza place. The people eating there were always an interesting mix of old timers and hipsters. Being next door to the BQE the location was a bit odd, but that was always the charm for me. Oddly enough I've never been to Nina's 1 which is located at 635 Meeker Ave.
I guess my new slices place will have to be Sal's Pizzeria at 544 Lorimer Street. Their thin crust Sicilian is pretty decent...