Well I really don’t agree with Hikaru Nakamura that Garry Kasparov’s main strength was the openings. I’d say it was his genius in playing dynamic positions, which when allied to a means of steering for them from the outset proved very difficult for his opponents to cope with. Of course it could be that Nakamura discounts much of Kasparov’s real strength because of his own great ability in complex positions.
Here’s a case in point, Kasparov’s brilliant win over Veselin Topalov. The opening could hardly be described as hot theory but he plays the middle game just brilliantly: