He is undoubtedly one of the most overrated players ever. All he did was reign(and reign supremely) for 3-4 years(70-72) and became World Champion. Then he saw Karpov and realized he could easily be overthrown as the World champion, made excuses to FIDE and fled the battle to protect his title.
If we are to consider the peak 3 years only of all the World Champions, I am not sure he would even be in top 5. He is like a one match wonder in any sports, somehow made a World record at debut, knew he wouldn't be able to carry on his performance, made excuse and fled.
Comparing him to likes of Kasparov, Karpov, Kramnik, Anand or even Polgar is an insult to them. At least they had the guts to keep fighting for their titles for years, in victory or in loss.
He was undoubtedly a brilliant chess player but 'best ever'? Please, maybe he was 'best' in the time span of 70-72, but not a 'best ever' by a long shot.
I have posted before, some strong chess players believe they got spanked by Fischer in online chess, way before chess engines were strong enough to have someone fake it.