Frankly, you might make a stronger argument about the laws of motion because Galileo made quite a lot of headway on those before Newton, and Galileo started from scratch. Newton reformulated them.
I'm not saying Leibnitz was a plagiarer. But he clearly had Newton's ideas, and they did exchange letters. Leibnitz would not likely have published what he did without Newton.
Personally, I think any one of Newton's contributions: gravity, laws of motion, or calculus were bigger deals than relativity, though I'm not suggesting that Einstein's contribution was small. It just wasn't as big as Newton's.
I would say that maybe Einstein's insight was more brilliant than Newton's were, but when you think of it, the laws of gravity really were a major insight.
But he clearly had Newton's ideas, and they did exchange letters. Leibnitz would not likely have published what he did without Newton.
There is nothing clear about it. The consensus is now rather the opposite.
Leibnitz published a lot on a variety of subjects. It is perfectly natural for him to have come up with the calculus in his other pursuits.
And a lot of his calculus was entirely unique and original to him.
And once again, Leibnitz and Newton never exchanged letters.
A good argument could be made that all of this came about because of Newton's notorious jealousy and crankiness. (He probably breathed too many mercury fumes. Cf. Faraday.)
If he hadn't made these accusations, it would have never come up. And Leibnitz wasn't the only person Newton made accusations against by a long shot.
"I would say that maybe Einstein's insight was more brilliant than Newton's were, but when you think of it, the laws of gravity really were a major insight."
This ranking business is childish.
Newton's Principia (written in 18 months) changed the world like nobody has before or since.
Before him there was no belief that the laws of nature were universal. That is, that things on Earth acted liked they did in the heavens and vice versa.
And theories, like Copernicus' were just fairytales. Myths. Likely stories. For there was no reason for things to be that way. No glue. (Such as gravity.)
Not to mention the small fact that Newtonian physics got us to the moon. Enstein and these other johnny come latelys had practically nothing to do with that-- or much else that is done in every day engineering and applied physics.
That is changing and will change more in the future. But still, there it is.