Champion sports bras are the best!  However, not all Champion bras are the same, and it can be a little hard to tell what's what with all the choices on Amazon.

This particular Champion sports bra is fantastic and I recommend it without reservation to any small or medium busted woman (say up to C cup) for running or any high impact activity.

It's (relatively) easy to get on and off, and the fact that it's seamless is really important if you're doing any time of extended exercise.  It also provides great support without being uncomfortable.