Another vote for Al Pacino, he is the very embodiment of cool!
And a nod to Takeshi Kitano for managing to play such cool characters despite being a funny little man with an amusing twitch in real life!
And, finally, I saw this guy sing the Wanderer from Siegfried in London a few months ago, and I finally understood what the phrase "commanding stage presence" actually means! He's a bit more "old world" model of masculinity than the other people named in this thread but, for me, he wins out because he manages to play it cool in spite of being OLDER than most of the people in this thread! Ladies and gentlemen I present to you:
John Tomlinson