Well, I could have picked some better opponents, but the battle between the celebrated Katz Deli and even more celebrated Carnegie Deli is absolutely no contest:
Katz (website, yelp, review here) doesn't disappoint. I've been there two times now and both have ended up with too much pastrami down my throat. Slightly roasted and never too much fat, Katz pastrami indeed deserves the credit. Highly recommended.
Carnegie Deli (website, yelp) on the other hand, serves enormous sandwiches that are all meat and no flavor. At least it was big and the pickles were good. Not recommended, unless you want to see a sandwich bigger than your face.
Katz hands down. And I need to try 2nd Ave Deli (website, yelp) at some point.