Lee Sin is just too strong. It would be inappropriate to balance up to him.
Much as I hate to say it, no, he's not.
He's too well rounded, not too strong. He does everything at a capable level. He's a decent tank, decent assassin, decent counterjungler, decent jungler, decent everything. He's got good mobility, good damage, good utility, and good CC. He has a good early game and can transition into a utility tank late game (which he's also good at.)
He's a jack of all trades that just happens to be very very good at those trades. He's not broken, he's just super well rounded.
To put it on a scale, Lee is a straight-B champion all across the board. Some champions might have As in certain categories but Cs or Ds in others. Lee is consistently good at basically everything (which isn't a bad thing.)
Nerfing him isn't a good answer. Buffing everyone else to his level might cause problems, but IMO it's healthier in the long run to have every jungler capable of handling Lee Sin without stomping everyone else or being outright OP to do so.
Lee himself isn't oppressive or particularly overpowered, he just dominates the meta by being what he is, a solid, always viable, always reliable champion with a high skill cap and high potential.
IMO buffing champions to be on Lee Sin's level is a better plan, though it's risky.