Unsophisticated observers watch Durant drop 35 while Curry scores 28 and they feel confident in their opinion that KD is the better player. But the analysis has already been done and Steph separated himself on that roster.

https://www.nbcsports.com/philadelphia/nba-insider-tom-haberstroh/steph-curry-warriors-breaking-point

In summary:

"The scoreboard with solo Durant: Minus-0.5 in 417 minutes. Yes, a negative point differential. And that makes sense given the loss of starpower, but Curry has shown he doesn’t need others to win. It gets even more stark when we isolate the Warriors’ four superstars. To fully capture the power of Curry, here’s the scoreboard with each Warriors solo act:

Lineup Net Rating
Solo Steph Plus-14.3
Solo Klay Plus-4.3
Solo KD Minus-0.5
Solo Dray Minus-2.0"

Durant is an unbelievable individual talent with arguably the best physical tools in the history of the game. But Steph Curry impacts winning at a level KD simply doesn't. And in the end, that means Steph a better basketball player.