There are two possible meanings of "powerful rationalist" here; "rationalist who has the most power" and "person who has the most of the kind of power that 'rationality' is meant to be".
I think the latter is more interesting; the first boils down to "find the most powerful person by conventional metrics who fits into the 'rationalist/rationalist-adjacent' cluster", any debate would be about how to define the borders of the cluster, which conventional measures of power are more important for "powerful", blah blah pointless semantics.
Lesswrong, at least in the early days, was about developing rationality as a superpower. We're long since disillusioned about how much of an advantage it can really be, but it probably at least does something interesting at the higher levels so… who has the highest mastery of our art, in the world? Has it done them any good?
The naive guess for "who" would be Eliezer himself, I mean he wrote the sequences. I think most people here would think it very dubious that he's the Most Rational Human, unless this turned into an Eliezer Yudkowsky Facts thread, but no-one in particular comes to mind when I try to think of who they'd put forth as a better example.