There are a few for me. Bobby Orr when he played for the Boston Bruins hockey team. He was poetry in motion.
Also on a basket ball court the way Magic Johnson moved the ball to insure the players could take the best advantage...beautiful.
On a tennis court it would be Roger Federer whose play is really quite often stunningly beautiful.
Golf? I'm not that conversant with good form but I do enjoy watching the tournaments. I leave that to you golfers to tell me whose form is pure gold! I am a huge fan of Bobby Jones who lived before my time and was an amateur his entire life. I think he was something very special. Also Seve Ballesteros. I saw a movie about him and I guess he was very very good.