lens for portraits nikon d90

I have a SB600 & a refurb SB800- I like them both.

I also have an AF NIKKOR 50mm f/1.8D lens and an AF-S DX NIKKOR 35mm f/1.8G.

I bought the 50mm first because it is inexpensive and it is a great lens too.

You neglect to mention your budget, but I think you can't go wrong with any of these....