Which train do you take to 44th Street and fifth Avenue in Manhattan?

It depends on where you're coming from. The closest subway stop is the 59th Street-Columbus Circle Station on the B and D (of the B-D-F-M, the orange line), the A and C (of the A-C-E, the blue line) and the 1train (the 2 late nights), of the 1-2-3, the red line.

The 59th Street-Columbus Circle Station is at Columbus Circle (obviously), which is 59th Street between 8th Avenue and Broadway.

But, unless one of these trains stops near your starting point, you'll have to transfer. It's impossible to tell you which train you'll have to transfer from, and where, without knowing your starting point.

Furthermore, it may be worth it to take another train to another stop that is not as close to 444 West 56th, to save yourself a transfer or two. For example, if the Q train (of the N-R-Q-W, the yellow line) is closest to your starting point, then it would be better to just take the Q to 57th Street-7th Avenue, and walk the rest of the way. It'll be a slightly longer walk than from the 59th Street-Columbus Circle stop, but then you don't have to make any transfers.