At the top of his game, Spassky held a FIDE rating of 2690 making him one of the best players of all time. In fact, Spassky was world chess champion from 1969 to 1972 – definitely making him amongst the best of his era.
Kasparov didn’t so much leave FIDE as he was thrown out of FIDE. He wanted a bigger say in the machinations of the governing body feeling, perhaps correctly, that players weren’t very well represented by the body but that the game’s money interests were. So, he formed his own competing association the Grandmasters Association and while this didn’t get him thrown out, the events of 1993 did.
The title of “super grandmaster” was invented in chess to distinguish the best of the best in the grandmaster class from other grandmasters. Super grandmasters have a FIDE ranking of over 2,700 and there are about 50 players in the world in this category, when compared to around 1,200 chess grandmasters!
Magnus Carlsen is not better at chess than a computer, despite his chess genius. A computer that plays chess at full capacity with no handicap, can look forward farther into hundreds of thousands of possibilities and compare them against each other. No human mind can make so many chess calculations so fast, not even Magnus Carlsen.