Here are three definitions of "topographic isolation".
The topographic isolation of a summit is the minimum distance to a point of equal elevation, representing a radius of dominance in which the peak is the highest point.
minimum distance to a point of higher elevation
The topographic isolation of a summit is the minimum horizontal (great circle) distance to the nearest point of higher elevation.
The gis.com site has a table of "The 25 Most Topographically Isolated Mountain Peaks on Earth". At the top of this table is Mount Everest with an isolation of 40, 008 km. I looked up that 40, 008 km is about the circumference of the Earth.
But I am not sure this choice agrees with either of the definitions above because one of the criterion of the definition is not satisfied: there is no other point with equal (or higher) elevation for Everest (or at least the highest point of it anyway).
Then again, I did grab rather informal definitions from some wiki pages, so perhaps there are other definitions for more technical work that resolve this apparent contradiction.
Miscellaneous afterthought: we should grant that we are talking about distances on a surface between peaks. There are taller mountains than Everest (e.g. Olympus Mons, but we are not considering the time-varying distance between planets in this question.