Stephen Lewis , Apr 18, 2006; 07:32 p.m.
If you're into digital, IMHO, the best would be a medium format with digital back...but that will set you back a real chunk of change, as I last looked the backs run around $10-15k. In "35mm" the Leica DMR would be next best, with a Canon and Nikon in 3rd place.
Honestly, though a good 4x5 film camera with tilt & shift is really hard to beat in portrait photography. If you need to ask, you need to spend some time with a skilled portrait photographer to see what is involved. (all opinions are humble, as I'm not a portrait photographer...merely an admirer).