How do you define a good liverwurst sandwich? What's on it? What kind of bread? How much do you care about the quality of the liverwurst?
I am partial to the Leberwurst Mit Zwiebelmarmelade (liverwurst with onion confit) sandwich at Cafe Sabarsky.
The homemade liverwurst at Cafe Katja is pretty good too, although I don't think it comes as a sandwich - you can put it on their bread though with their very good mustard.
Loreley serves a liverwurst sandwich - very straightforward but good quality. Not exactly upscale liverwurst, but fine.
Eisenberg's makes a decent liverwurst sandwich - you can order it however you want.
And there's always reliably good liverwurst at Schaller & Weber, and they do make sandwiches. I am partial to their house-made liverwurst which is only sold at the store on 2nd Ave/86th St., although their braunschweiger is good too, if a bit creamy for my tastes.