This debate has been done to death...but to put it briefly. Nearly every team he's been a part of, he's been the most successful driver. Even when Rosberg beat him to the championship that year at Mercedes, it was more of a case of Hamilton having rotten luck with engines etc, than Rosberg being the better driver.
He's definitely worthy of SPOTY this year. What he's done in F1 throughout his career will take some beating.
It's only a matter of time before he takes over Schumacher's 'Most F1 Championships' title.