These gyms are all close together in the Chelsea area. Try a free workout at each then decide:
24-7, 14th st. & 6th:
Inexpensive, somewhat crowded, good equipment if heavily used/worn.
Steel Gym on 23rd between 6th and 7th.
Kinda small, been there forever under different names.
David Barton gym YMCA building 23rd between 7th & 8th.
19th St. gym between 5th & 6th
I like both of these gyms for the energy and equipment. Very gay, but they're good gyms if you can overlook this.
Chelsea Piers, 23rd st. west & west side highway:
Huge, worth taking a look for the variety.
Mid-town west:
Gold's, 54th W.
Mid-City Gym 49th between 7th & 8th. Classic old-school NY original. Good equipment, a little grungy, lots of history.
Syosset's a nice schlepp from NY; unless there's a compelling reason to go there in particular, don't bother. The suburbam gyms have more space but it's not a crucial difference.