well here's someone no one has spoken about-
Ruslan Ponomariov (Yes, this was during the split years, with PCA and FIDE)
He was the youngest GM (at his time, at the age of 14)
In the 2002 World championships, he was seeded 19th. With players Anand,Adams,Morozevich,Ivanchuk,Leko,Bareev,Gelfand,Topalov,Shirov,Kasimdzhanov,Smirin Ilya,Khalifman,Van Wely,Georgiev,Svidler,Karpov,Dreev and Judit Polgar ranked above him.
Ponomariov's group wasn't the easiest either.
It had 3rd seed Morozevich,14th seed Georgiev and other notable players, like Tiviakov,Krishnan Sasikiran,Mikhail Gurevich,Nispeanu etc.
Ponomariov beat Georgiev (2-0) ,Tiviakov (peak 2699) and Morozevich to qualify
Then beat 6th seed Bareev 2-0 on tiebrakes (3-1, unbeaten)
Beat 15th seed Svidler 2.5-1.5 with a win as black (again, umbeaten)
Then held draws to Ivanchuk with Black,and beat him twice as white to be the FIDE world chess champion at the age of 18 Years and 104 days, which is to date, the youngest World Champion ever.
In 2004 and 2010, he won the olympiad with Ukraine. He was the runner-up at the World Cup 2005 and 2009. And in 2005, Ponamoriov beat fritz 9 1-0, being one the last instance were a human beat a chess machine under tournament conditions.
He's had a peak rating of 2764, and is one of the world's leading experts of French Defense.
But what I find surprising is that not many know that Ponomariov is a World Champion.