If this letter from me is untimely, you may punish me by not replying: I shall be satisfied simply to have expressed my feelings towards you. I think I have read most of your works, partly in separate volumes and partly in the collected edition, and I freely confess that I have profited from few other works of our age as much as I have from yours. I am not in the habit of flattering, but everyone who has been able to understand your writings on political theory [civili doctrina] agrees with me that nothing can possibly be added to the clarity of their arguments, which is so admirable when they are expressed so concisely.
Leibniz to Hobbes, 13/23 July 1670, in Hobbes, The Correspondence Volume II, pp. 713/716-7.
It’s not clear that this actually ever reached Hobbes: there is certainly no known reply. So Leibniz writes again a few years later:
in your little book De cive you seem to have surpassed yourself in the strength of your reasoning and the weight of your opinions, so that one might think you were giving the pronouncements of an oracle rather than handing down the theories of a teacher.
Leibniz to Hobbes, 1674?, in Hobbes, The Correspondence Volume II, p. 731/733.
Leibniz was only 24 when the first letter was sent, but even so…