I love the 72 needle cylinder for my socks, but most of the gift knitting I do is on the 60 needle cylinder -- I'm _always_ procrastinating, and the 60's just a little bit faster (less stitches to kitchener, too, LOL, but who's counting??)
For my mom I use an 80, though, to get a larger leg with regular sock yarns. And for my daughter I use a 54 cranked to tightest tension to make them small enough (she's 7).
I guess if I had to have just one, I'd use a 60 and not use commercial sock yarns (the original plan _was_ to make socks with my handspun, ROTFLOL)
(question came up on sockknittingmachines, 19April2007)