My neighborhood standby is Wu Liang Ye on 48th St (try the cold conch and crab dishes.. yum) and Ollie's is surprisingly authentic despite its chain-status.
Szechuan Gourmet. The spiciest I've found in Manhattan. Get anything with 3-4 chili peppers next to them on the menu, like the mapo tofu, braised [insert meat here] with napa and chili, dumplings in chili-soy, or cumin lamb. Great. I live right next to a Grand Sichuan and prefer SG's food.