I think a lot depends on what you mean by greatest ever. If you mean who would win in a direct 1v1 World Championship match then I think it would be Garry Kasparov. Magnus Carlsen #2, but he's shown he isn't willing to work hard enough to enter and win World Championships over and over again like other great champions did, so Kasparov>Carlsen for mental toughness and being more motivated for very long classical tournaments. Bobby Fischer was a bit like Carlsen because he refused to play against the great Anatoly Karpov when offered, making up lots of crazy excuses and demands.
The thing is with questions like this, we will never know! 
Hi guys thanks for your reply!
For me guys, and my idol Wesley So's too, it's American Bobby Fischer!!!
He was the enigmatic, supernatural, once in a lifetime Don Quixote of chess who travelled all by himself thousands of miles away to single handedly demolish and bring down the almighty Russian Chess Empire!!!
After winning the 1972 World Chess Championship in Iceland, he quit chess and rode off to the sunset.
He also invented my most favorite chess game Fischer's Random 960!
We will never ever see someone like Fischer again.