What is best lens to use for natural lighting portraits?

I use a nikon d70s and do mostly model portfolio and band promo pics u can see examples of my work here:
http://s17.photobucket.com/albums/b70/levimckeellive/photography/?action=view&current=1165950568.pbw

I was wondering if any one had an idea for the best reasonably priced lens I should buy for these types pictures.

P.s. I have no lighting equipment or flash yet, keep that in mind :)