I haven't done a lot of gv'ing, at least until recently, but seems to me the M8 is hands down the fastest tank. I tried one the other day and I believe I was going around 50mph, or faster. Never came close to that in any other tank.
They seem to be thin skinned, though. I've tracked them and killed them from my trusty Nikki, unlike other tank type vehicles. Still, if I need to get somewhere fast- to defend a town, for instance- my choice would be the M8 or M16.