"Best" is very subjective and generally also depends on how you are intending to use it!!
I have 4 lenses in my "usual" kit, and depending on the situation, I use all 4 of them for portraits!
Nikkor AF-S 17-55/2.8 DX - Probably my most used lens as the zoom range is very versatile. Can do medium sized group shots, and does very nicely at portraits at 55mm. Not my first choice portrait lens though
Nikkor AF-D 85/1.4 - I love this one for portraits. Legendary bokeh and razor sharp images. Nice to play at f/1.4, but I get my nicest results at about f/2
Nikkor AF-S 70-200VR - If I have the space, this one is brilliant. I think I actually use it more for portraits than the 85 as I am more likely to have it with me (My most used combo that I drag around is the 17-55 and 70-200).
Tokina 12-24/4 - You're really not supposed to use ultrawide lenses for portraits, but what are rules for if they're not for breaking? Funny enough, my very very most favourite portrait I've ever taken (possibly even my favourite shot fullstop) was taken with this lens at 12mm.
In short, I don't think you can go wrong with any of the following:
Personal Experience: 50/1.8, 85/1.8, Tamron 28-75/2.8, 70-200VR, 85/1.4 (the last 2 particularly good)
Hearsay: 105mm, 135mm, 200/2
If you are looking for a straight replacement for the Canon 85/1.2, the closest bet would be the 85/1.4