Best full size pickup for what task?
Heavy duty stuff like towing and hauling all the time, I'll have to give a nod to Ford - they have the best super duty chassis. As much as I hate working on the freeking things. GM doesn't have a 1 ton truck anymore that has a straight front axle - it's all independant suspension now. Great for that "car like" ride that they advertise, but sucks for heavy duty applications. We run into problems where I work (fleet wholesaler), they just want to pick the nose off the ground when you load them up.
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Mostly just driving back and forth ocasionally doing some hauling or towing? Like a 1/2 ton pickup? Go with GM - they have the best powertrains, especially with the Duramax and Allison combo - though they're only available in the 3/4 and 1 ton trucks. I still like the 5.3 and 6.0 gas engines too - despite the cold start knock they're known for.
Dodge I despise and they don't know how to make a car, much less a truck - pitifull.
Toyota and Nissan are coming up. They already build cars better than we do, once they figure out how to build a heavy duty chassis they'll be all set and will take a significant chunk of the market. Though they'll probably never own it, like they own the car market, because well...Bubba loves his 'merican built Ford pickem up truck.