The 2nd Ave Deli, naturally located between 3rd and Lex, is my favorite jewish deli in the city, with the possible exception of Barney Greengrass uptown. And I've been to them all. Unlike Carnegie or Katz's, this place is always busy but never chaotic, and largely tourist free. I grew up just outside of Philadelphia and, as a child, ate at least once a week at Hymie's Deli in Bala Cynwyd. The 2nd Ave Deli always reminds me of Hymie's: the staff are warm but efficient, the matzo ball soup at once reminiscent of and light-years better than my grandmother's, and the pickles free, many, and delicious. Make sure to take along someone willing to order the hot pastrami sandwich on rye. It's probably ten thousand calories, but it's wonderful.
162 E. 33rd St. (b/w 3rd and Lex)
hot pastrami on rye
smoked whitefish on a toasted bagel