50mm (f/1.8 and f/1.4) is ideal for general portraiture. Great focal length - not too short (to distort), not too long (so you can't work intimately). Great optics, good build, good AF.
24-70mm f/2.8 speaks for itself as a very, very good general zoom, although the wide end isn't that wide on DX. Optically ace. f/2.8-4 version is old, apparently pretty good but not a patch on its big bro. But it's much cheaper.
105mm VR - I have no experience with this lens. Probably good for snatch shots at events etc because of the length and the Vr would help in low light.
Don't know the Sigma but probably a decent general zoom for little money.
Tamron 90mm gets lots of thumbs up from macro types and will probably work well as a portrait lens.
Nikon 85mm - great lens for all-round portrait thanks to A) fast aperture B) great optics. I have the pre-'D' version and it's amazingly sharp. It's my general grab shot lens, especially in low light, but I also like it for facial portraiture because of the focal length that helps exaggerate the DoF. Here's one I did on mine:
Trev by
Pat MacInnes, on Flickr
Personally, I'd think about the kind of portraiture you want to do. A zoom gives overall flexibility, especially when you want a variety of shots, but the compromise is the max aperture will be f/2.8, although that's still pretty shallow DoF. A prime will give you top-drawer optics for relatively little money and a much faster maximum aperture (great in low light), but you have a fixed focal length so that's something you'll have to live with. It can help to focus your creativity but in some instances, it's a hinderance because you need physical space to zoom out.
I think the f/1.8 50mm is a great buy because it's cheap, has good IQ and offers a fast aperture. It's budget end but doesn't produce budget results and isn't a massive investment if portraiture doesn't end up floating your boat....
