Crosby is up there. Not sure that he's one of the very very best captains ever (as many said, it's impossbile for us to know from the outside), but we've heard enough stories and testimonials from teammates and coaches and such about his leadership that he should be among some of the popular names in consideration here.
I had voted for Beliveau when poll was made a few years ago, sticking to it. Another name I like is Yzerman.