Greatest chess player never to have become World Champion

The rise of Viswanathan Anand to number 1 in the world rankings gives an excellent excuse to revive the debate as to who is the greatest chess player never to have become World Champion.

I'm going to list 4 contenders, and categorically state that no-one else merits a mention. They are Paul Keres, David Bronstein, Viktor Korchnoi and, of course, Vishy Anand himself. I guess in the case of Vishy and even Viktor (we can hope) they could still become champion.

Keres, Bronstein and Korchnoi were all hampered in their bids to become World Champion by the Soviet authorities; Keres allegedly even having to reach a deal with Stalin to avoid being executed.

Paul Keres, an Estonian, was probably the strongest player in the world in the late 30's and throughout the 40's. His progress to the title being hampered by the outbreak of the second world war, and then afterwards by the rise of first Botvinnik, and then Smyslov and Tal. Allegedly, Keres was spared his life after the war (during the Soviet reprisals against the Baltic States) in return for 'throwing' the 1948 World Championship tournament to Botvinnik. He died suddenly of a heart attack in 1975, and was given a state funeral in Talinn attended by 100,000 people.

David Bronstein, as well, was allegedly pressured by the Soviet authorities into not winning his 1951 World Championship match with Botvinnik. Bronstein led with 2 games to go, but played badly in the penultimate game and the match finished in a 12-12 tie, allowing Botvinnik to keep his title. He makes my list not just for drawing this match, but for continuing to play at a high level into old age, writing about and contributing so much to the theory and enjoyment of chess, having a James Bond character based on him, and best of all we shared a beer and briefly played for the same team. David unfortunately passed away at the end of last year.

No-one's tussles with the Soviets are better documented than Viktor Korchnoi's. A survivor of the siege of Leningrad, he defected in 1976 in Amsterdam. He contested 2 (effectively 3 if you include the '74 Candidates final) World Championship matches with Karpov, losing by the odd game in '74 and '78. In 1974 he was allegedly beaten up by authority thugs during the match, and in '78 had to beat all the Soviet guard (Polugayevsky, arch enemy Petrosian, and Spassky) on his way to the title match with Karpov. He was amongst the world's elite for 4 decades, and still ranks in the top 100 at the age of 75.

Which brings up back to Vishy Anand. In 1993, he led his World Championship challenge against Kasparov for a long time, but finally cracked and Kasparov finished the match remorselessly. As a youngster he was famed for the fluency and speed of his play, often finishing off strong grandmasters in a matter of minutes. He seemed to lose his edge after the match with Kasparov, but is experiencing a resurgence and is now only the sixth player to have topped the FIDE rating list since its inception in 1970.

Ok, so who've I missed off, and of the 4 players, who was the greatest chess player never to have become World Champion?