Slice of New York (theres one on both sides of you in Seal Beach and Huntington Beach) IMO its as good as you'll get in the area as far as NY style goes.
If you want strictly "in" sunset, Stoneys is pretty good and sells by the slice.
If you liked A Slice of New York and not looking for delivery, you should try their seal beach location sometime. I think it’s way better and not too far of a drive from the HB location. It’s probably closest to NY pizza I’ve had in LA/OC though there’s still a few places I need to try.