This really depends on your work and the ranges you work in but from a strictly thickness and diameter standpoint, the follow would provide the greatest range with the least number of tests.
A piece of pipe with a thickness of 0.5" or greater, so 2.5" NPS Sch. XXH (0.552") or 8" NPS Sch. 80 (0.5") for rotator work. This would provide you with the maximum thickness range.
To get the smaller diameter, a 1/2" sch. 40 would work. Using Table QW-452.1(b) note 2, the diameter range from 1/2" up to the diameter of the piece used to qualify your welder can be used on the small diameter. So with these two test your welder is qualified for 1/2" diameter, unlimited thickness.