Originally posted by Adam DA* 50-135mm.
I would suggest this one, as it gives you some flexibility and is plenty crisp.
Keep in mind that getting crisp, colourful,
good photos is the result of many hours of studying, years of experiences, experienced models, and a lot of support equipment.
If you are on a tight budget, look at Pentax DA 50mm f1.8. No zoom, but nice and sharp. Choose Av mode, go down to f2-f3,2, and you should be able to get fairly nice portraits. That said, the DA* 55mm is practically made for portraits. As is the FA 77mm limited. Sigma 85mm and Samyang 85mm are portrait lenses, too. Samyang has no AF and might be difficult to focus. Notice none of these lenses have a zoom range. This is because for top notch quality, you don't get compromises in the optical department. This also means the lenses are far more challenging to use.
Good luck