I'm a Topeak Road Morph fan. I mounted one on the top tube for a while, until I took the pump off to mount the bike on a bike carrier and drove off leaving the pump somewhere along the way. Its replacement sits happily on the left seat stay, out of the way of the rack and panniers, out of the way of the bike carrier, but quickly ready for use when it's needed.

But here's hoping you'll never need a pump on the road!