Somewhere around 1500 to 5000, but as as others have indicated it really depends on what are your criteria for "overall ability."
For sprinters it's the 200, or maybe even 300 m. But those who are world class or national class will flame out past 400 or 500. So does that account for overall ability?
Likewise, the 800 is a brutal event no doubt but I've seen 1:48-49 runners struggle to break 5:50 pace for 8K.
The really great ones are like Rod Dixon, who did it all 3:33 1500, 13:17 5000, an Olympic medal in the 1500, and NYC Marathon win in 2:08:59. Plus he had a long career.
In the modern era--or any era--Geb would be the GOAT. Good 1500 speed (3:33) and a 2:04 marathoner. Are there any sub 3:30/3:50 for 1500 mile, sub 12:50 5000, and sub 2:05 marathoners? Maybe Mo Farah someday.
Kind of taking this on a different tangent, but for distance running the 1500-5000 sets the groundwork for others. Plus some of the really fast ones can do a sub 50 in the 400, and in the 10s for 100 m.