The yardstick of a great broadcaster for me is someone who can get you excited about a sport that you don't basically give a toss about.
Peter O'Sullevan is alone in that category. I'm not interested in horse racing, whether flat or steeplechase (despite, or perhaps because of, the fact that it was my father's great passion, and the only sport he followed). But I always used to get excited along with Peter O'Sullevan. He was like a kid who actually made me laugh. And I still get goosebumps when I listen to those three Aintree triumphs by Red Rum. The last one is just staggering.