NBA.com has clutch shooting statistics for every season dating back to 1997.
So which player is the king of clutch?
I wanted a large sample size, so I chose LeBron, Kobe, KD and Steph. I compiled numbers for both the regular season and playoffs.
What we are looking at here is total points scored in the clutch, as well as total field goals made, total field goal attempts, and field goal shooting percentage.
Here are the results for the regular season:

LeBron is unquestionably the best of the four in the clutch in the regular season. His 2,534 clutch points are the most, as are his 801 made field goals. Overall, he shoots 45.9% in the clutch for his career in the regular season.
Steph comes in second at 43%, KD comes in third at 40.8%, and Kobe is in 4th at 39.9%. However, Kobe has the highest clutch scoring average per game at 3.707 vs. LeBron’s 3.662.
What about the playoffs?
The playoffs are tougher to evaluate because there is a smaller sample size, but here are the results:

LeBron has the most points and made field goals in the clutch, but he also has played the most playoff games. His field goal shooting average in the clutch is 40.9%, which is slightly behind KD’s 41.1%.
Kobe comes in 3rd at 39.6% playoff clutch shooting, and Curry is 4th at 39.2%.
But the thing is, the sample sizes are so small it’s really hard to draw too much from these numbers.
For example, the difference between KD being first place in playoff clutch shooting percentage is 4 shots. If he had made 77 out of 197 instead of 81 out of 197, he would have a 39.1% field goal percentage.
If LeBron was simply 140 out of 340 instead of 139 out of 340, he would have the highest field goal percentage of the four players by a tenth.
So what I would say is that LeBron, Kobe, KD and Steph are basically tied when it comes to clutch shooting in the playoffs. There isn’t really a big enough difference between them to definitively say who is the best of the four and who is the worst. They’re pretty much dead even.
And another thing with the small sample size of the playoffs: we are assuming that shot/look quality is equal among the four guys when it very likely isn’t. Kobe was known for making tough shots. LeBron is consistently mobbed by defenders and doesn’t get the whistle anymore.
The bottom line, though, is that it’s impossible to tell who’s the better playoff clutch shooter. When the sample size is this small, there’s a lot of variability. And not all clutch shot attempts are created equally.
So LeBron is the clutch king of the regular season, and there’s no clear winner for the playoffs.
Again, these numbers only go back to 1997 so we can’t look at MJ’s entire career, or Bird or Magic or any of these historic greats.