I personally find Tal, Larsen and Rubinstein hold the most interest for me, but I can't necessarily say why. It's partly due to their personalities and attitudes to the game, as well as their game and personal history, and chess skills.
I wonder if I'm the only one on this forum owning Larsen's "50 Udvalgte Partier"? 
So, who are your favorite top 3 GM's of all time? I consider Tal, Fischer and Kasparov to be the greatest ambasadors of the Game because no one has ever, in my opinion, contributed so much to the global popularity of chess as those three gentlemen...