My favorite climber that I've seen is Indurain. We rarely got to see his true capability, but when he showed it, it was amazing. He was graceful, powerful, and ultra-smooth; true class.
One of my favorite stages of the Tour remains his ride to La Plagne in '95. Zulle is up the road and Indurain realizes he has to catch him. Seemingly without effort, he just rolls up the pace, never even gets out of the saddle and sheds the best 'climbers' in the business. I love watching that video of Indurain on the front, smooth as ever, and see Chiappuci, Rominger, Virenque, Tonkov all suffer then explode. What makes him a champion of the sport is he could have done this more frequently but didn't.