I think that part of what was marvellous about Newton (Einstein, too) was how many things he did. As you say, it seems plausible any individual thing Newton did would have been done by someone else—probably the same is true of anyone—but it seems unlikely that any one person could have done everything Newton did.
I suspect that if you asked the HN crowd for their guess at the smartest person, Mozart (thanks to Hofstadter) and von Neumann would come out on top (though I might just be biased towards whom I'd answer). Tao is a likely candidate for living person.
In terms of impact, Newton is truly stand-out, though I think when it comes to math, his most significant contribution was calculus, and Leibniz came up with that independently at the same time, and we now use his version instead of Newton.
Few in the HN crowd understand the mathematics Grothendieck worked in and therefore cannot properly assess his contributions and intellect due to their unfamiliarity with his work.
von Neumann had savant like intelligence, but Grothendieck was in an unmatched league of his own in terms of abstractions.
I was specifically thinking about physicists in that case, though I guess Neil deGrasse Tyson and Carl Sagan fall under that umbrella.
I also don't know a ton about him, but as someone in the math scene, he's the only modern-day mathematician I hear about with any regularity.
If you partially submerge a stick into water, it appears as if the stick is broken. When a physicist first sees this phenomenon, their reaction is: space must be warped/contracted/broken! It's a strange kind of intuition.
One thing that does amaze me though, is his unbelievable capacity for intuition.