With portraits, you want a large maximum aperture both to get good results in low light and to get good background blur, which focuses the viewer's attention on your subject. It looks like price is an important factor for you, so to get a fast lens (say, f/2 or faster) at a price less than $500, you're looking at prime lenses, not zooms and you can forget about "L" glass. I would agree with others that the 50mm f/1.8 is the best value you can get - a perfectly usable lens for under $100. If you have several hundred to spend, you can look at the faster 50mm f/1.4 or go for a longer focal length with the 85mm f/1.8. The longer focal length not only allows you to stand further away from your subject (less intrusive, as someone already mentioned) but it also flattens features more, which is usually a good thing in portrait photography. I don't think Sigma or Tamron makes anything faster than f/2.8, so you're probably better off sticking with Canon. If you want good lens reviews, check out http://www.fredmiranda.com.