I cannot declare Kobe the most clutch ever, as I've not seen enough of historical players to make that call.
What I can say is that while I've been a Celtics fan (which was basically since KG came here - as he was my favourite player) there is no opponent who has scared me more at the end of games then Kobe.
The big reason Kobe scared me so much was his offensive versatility. Kobe was one of those rare offensive players who could score from anywhere and everywhere.
He could post, he could drive, he had an amazing midrange game, he could hit the three, he could score in transition and he could get to the line - where he shot a high percentage. His jumper was especially scary because he could take the pull up, he could spot up, he could shoot off screens, he could hit turnarounds, he could hit fadeaways - there wasn't a single type of shot Kobe could not hit.
When Lebron has the ball, you can try to force him into taking a long jumper, and you improve your odds of him missing. When Shaq had the ball, you could foul him and force him to hit free throws. When Curry has the ball you force him to put it on the floor, smothering him with full court pressure and maybe force him into a turnover. Even Jordan - for most of his career he was a pretty poor three point shooter, so if you could somehow force him into a three you had a shot.
With Kobe, there was no game plan. You could not force Kobe into an uncomfortable shot, because there is no such thing as an uncomfortable shot for Kobe. For Kobe, a turnaround, fadeaway 26 footer with a hand in his face is pretty much a routine jumper. You can't play Kobe close, you can't sag off him. You can't foul him. You can't force him left or force him right. You can't force him into a difficult contested midrange jumper, because he loves those. You can't give him layups, you can't let him get on the fast break, you can't give him a three, you can't force him into the post, and you can't force him to drive and then send a big over to block him off. You can't even double or triple team him because he was such a good passer that he could find the open guy.
Kobe, when in the mood, was absolutely indefensible.
Scarier then that...when it was crunch time, he was ALWAYS in the mood. You can't ever shake Kobe's confidence. You can't even scare him away from taking a shot. No cold streak is going to make him second guess himself. The man is a stone cold killer.
So for the above reasons, no opponent has ever scared me more then Kobe.
If I had to name the guys who would come after him, it would probably be Melo and Dirki when they were in their primes. Melo is right up there with Kobe as one of the most versatile scorers the NBA has ever seen. Dirk is not quite as versatile, but his size combined with his skill as a shooter make him almost impossible to spot as well.
There may have been players who were more consistent offensively then Kobe. There may have been players who were more efficient offensively then Kobe. There may have been players who were more fundamentally sound, and who made better decisions with the ball.
But I don't know if I have ever watched a player who was as utterly capable / unstoppable as scorer (at his best) as Kobe was.
