GMC Sierra or Dodge Ram
If it's gas powered, GMC. Chevy makes reliable V8s that are realativly easy to work on, and parts are everywhere. If it's diesel, Dodge, the Inline 6 Cummins has proven to be bullet proof. Especially if it's a older 5.9 Cummins. They have not only proven reliable in trucks, but farm equipment, construction equipment, hell the British military used a 5.9 Cummins to power a tank.