Lebron is on that level, however, he will never be viewed as the best similar to how Michael Jordan is viewed as the best. Hard hehehe.
It can't be avoided that there are some comparison, Leonard is good because he won the championship 2 times already and until now he is still an underrated player. Maybe because he wasn't recognize as a star when he enters in the NBA unlike Lebron James that he knew already that he can be like Kobe when he first entered in the NBA.
Leonard is still 28 years old and he will continue to develop himself, and though he doesn't have a fancy moves like Lebron and Kobe, but look at his stats, he is so consistent, he come up clutch in different important occasions and that's the quality of a real star.
Jordan won his first ring at age 28, so there's something that we can compare with. However, we know that Jordan won 6 rings so I don't think that he can be in the discussion for the GOAT at this time. If he continues to win with Clippers then maybe just maybe we can compare him with the likes of Lebron James and Kobe Bryant.
All of Lebron's NBA Finals win happened because he pair with another NBA star or stars ,but Leonard did not choose an NBA star teammates, he played his game with whoever his teammates are and as we saw with the way he played with the Raptors, he does not demand the ball all the time, he trust his teammates, in short, he made the Raptors the best team in the NBA which was previously swept by team Lebron in the playoffs when he was still with the Miami Heat.
He didn't chose because he doesn't have the luxury, he was traded to after playing with Spurs and at that time he had the big 3, Duncan, Manu and Parker. However, since he has the bargaining power now chooses to move to a team with other star as well, Paul George. So that argument that he didn't choose star teammates is flawed. But I will give it to him for winning a ring in Toronto though. And that is why we can have a discussion about him being one of the best if he won another NBA ring with Clippers since that division is pretty stack up at this point.