You ask me, the most capable leader Persia had in the 18th Century was Karim Khan Zand - the guy who did the most to clean up after Nadir Shah.
For what Nadir built to survive, what was really needed was a competent heir with a decent amount of legitimacy, so that the civil war that comes after Nadir's death isn't too destructive (hard to see Persia avoiding a civil war once Nadir died). Frankly, that is asking a bit much though. All of the likely contenders either are not very competent, or lacked legitimacy (so would only come to control Persia by climbing a hill of corpses).
I've always wondered what would have happened if Nadir died early - on the way back from India say. Then, Persia is spared the tyranny and expense of his later reign, and the legitimacy of his successors is boosted by the good memories of the great Shah Nadir. Also, his successors have the benefit of all the loot taken during the Indian campaign. Unfortunately, his heir at this point, Reza Qoli Mirza is not spoken well of by the histories (though, some of that may be propaganda), and even if history misrepresents him, he still bears the stain of having the Safavids murdered, so would surely face opposition from Safavid legitimists.
fasquardon