Avetik Grigroyan, because he turned to coaching. haha jokes... Yes I think the names many mention are Keres, Korchnoi, and Rubinstein. Bronstein and Leko were on the cusp... very close in their matches. As to why, I am not knowledgeable enough to say whether it was something in their chess, mentality, circumstances etc. Interestingly, it seems Geller has a plus score against many of the world champions individually.. but of course this is different than going through the world champ cycle and matches.