We must remember, it is called a "transport". It was designed to move a mass amount of troops not a primary assault weapon.
Well, with only room for 40 troops inside though (If we go by "official" sources that claim that #) it moves very few troops really.
Plus the "Armored" side of things seems to indicate it's meant to trudge into heavy combat situations as well. The AT-ST is also labeled "Transport" in its title but it only carries 2 crew and a maximum of 2 extra folks crammed into the cockpit. So it's not really a "transport" either.
At the battle of Hoth we see the weapons of an AT-AT at full potential, and they do a substntial ammount of damage.
A heavy laser cannon capable of charging up to destroy an armored structure with one shot or fire rapid/less powerful shots on shorter distances. And then 2 pivoting anti-personnel blasters that can cover most any arc the vehicle needs defended... That's pretty heavy duty weaponry to still be hauling 40 troops inside its belly too.
I think the main thing to remember here is the seemingly staggered formations the Empire employs for its walkers. They're not generally meant to be out on their own, but rather they're intended to be in support for each other (If we look at things from an EU Perspective).
The AT-AT is the main "brute" of an Imperial armored column, with AT-ST's in flanking support. The AT-ST's have anti-personnel walkers as well (AT-PT's) to mop up the little stuff, and infantry in support of those. And in the rear lines AT-AA's (Again this is if we follow some EU establishment).
While each walker can function on their own they function better together. AT-ST's appearing to be the most versatile of the group with AT-AT's a close second (AT-AT's performed some nimble tasks that a vehicle that size wouldn't be expected to at Hoth as I recall).
The best, except for certain situations though, would be the AT-AT. In ESB they're seen pinpoint shooting individual infantry, destroying large structures, shooting down fast/difficult aircraft, and all the while they advanced rather rapidly and with minimal support (One AT-ST is seen, it can be assumed more were there, but it wasn't a complete structured/full assault since time was of the essence).
The AT-AT dominated the battle so I really think its front line importance is combat more than transport. Plus again, 40 troops is really not much at all... A platoon at most.
I think the Transport title's very misleading, a nd more a generic term used for many of the Imperial land vehicles with some transporting capabilities.