Oh boy! How do you nominate just one?
Here's an offbeat choice and somebody that few people today will know. He was a Maltese tenor named Oreste Kirkop who starred in The Vagabond King. He was superb.
But for sheer imperturbability you couldn't go past Luciano Pavarotti. I was driving home in heavy traffic one afternoon listening to the Three Tenors concert when some moron who had to have been either drunk or high, drove at speed from a side road,, straight into the stream of cars. I stood on my brakes and managed to miss him without being wiped out by the guy behind me but I had to take my hot off to Pavarotti. He didn't miss a beat.