Authentic will mean different things to different people. But for quality, Sunnyside has Philomena’s, which is getting a lot of deserved attention right now. Great slices, both round and square.
If “authentic” means the way it was in the 70’s and 80’s, you’re probably fine to just stop in any small shop with a beat up looking awning that sells by the slice. If you want old school standouts, the thread Jon linked to has excellent suggestions that will take some traveling to get to.