When I began running, finding a sports bra that did not chaff when running long distances was tough. Problem areas for me are usually right under the arms and in the front along the bottom of the bra. A few years ago I purchased a seamless sports bra from Champion and I have never bought another brand since then. These bras have no seams to rub against your skin. They do not entirely eliminate chaffing, but when used in combination with a sports lube it sure does help. The one thing I would change about these bras is that they have a small o-ring that connects the strap to the body of the bra. Normally these rings serve to adjust the length of the strap, but they serve no such function in this bra. So, there is this weird plastic ring right in the back of an otherwise seamless, perfect sports bra. I've never chaffed because of it, but sometimes I can feel it and it leaves me wondering what it's doing there in the first place.
The style I have work well for small to medium cup sizes. Large-busted women (larger than a C) may not get enough support from this sports bra.
I have four or five of these bras and they wash up really well. I have run many miles wearing these and can highly recommend them to anyone needing a great, supportive sports bra that does not chaff.