Nowadays, it seems that my main criterion for choosing a knife is that it will comfortably fit in the watch pockets of my 501s and chinos. for the past several months it's been this:
but I have carried up to a 3318HE without any complaints. The key for me is a slim profile + rounded bolsters.
"Swiftly he got out his Case XX, still secure in the watch pocket of his Levi’s, thanking God for a good Case knife that would hold an edge all down the many years and thanking God also for his own stubborn ways that made a small knife as much a part of daily dressing as boots and socks." S. Hunter