Anything up to a 1 ton would be a good start, plenty of parts for original and easy upgrades if desired.
If you decide on a 1.5 or 2 ton they're also good but pretty slow as they came and if speed is desired take a lot of swapping and work to get better brakes and faster gears.
As Ed asked, what do you want a truck for?
Figure that out, make a plan on how to get there, and find a truck that fits.
