I suspect that it is different for most guys; shape, material, neck and base are all important considerations.
For me, one to wear long periods is this one:
That said, this summer during a hot day I was wearing cutoffs. I had an ordinary medium size buttplug in me, but hadn't tightened my half-harness to keep the plug secured. Anyways, while walking the plug kept popping out and than pushed back in by the harness. It felt like being fucked while walking on the sidewalk, in public!