Depends on what you want to do...
If you plan to stay below exposure times where reciprocity failure kicks in (so below about 1 sec), HP5 pushed to IE 3200 is probably your best choice.
However, as soon as you get in the reciprocity failure ballpark (seconds, minutes, hours), TMax400 will far outperform any other film, as I discovered recently by experience and after some advice of other APUG members. It even beats Fuji Neopan Acros 100 in terms of necessary exposure times to get a certain film density (at least up to 8 hours exposure times as I tested), although Acros 100 seems to have the best reciprocity characteristics in terms of the number of stops lost per time unit. However, up to 8 hours real exposure times, TMax 400 will not loose it's entire 2 stop advantage compared to Acros 100, although at 8 hours film densities of these two films are getting awfully close...
Pushing simply does not work when reciprocity failure is an issue, since no stable developable latent image is formed, contrary to when you have "normal" exposure times. A nice article about this is:
http://silvergrain.org/Photo-Tech/reciprocity.html
See also this thread I started:
(there was a url link here which no longer exists)