[joke]I'd pick whatever Dave's picking, in the hopes that he's figured out how to turn one into a boat propeller.[/joke]
Seriously, though if I were presented with those four options in a game similar to recent years, where we haven't had to climb over a six-inch wall or anything, I'd pick the 6" with a bore for bearings. (That said, I'm being ridiculously cost-conscious as I plot and scheme this year; $59 each inclines me towards mixing them with Skyways as insurance in case my driver
pulls a 1902 and flies off the ramp.)