He’s the best distance decathlete. By that his combination of track, XC, and marathon performances together. However, he was not the best individual in track, XC, or marathoner.
Best XC = Paul Tergat (dominate for many many years)
Best Track = Geb (His 2000 10k gold against Tergat is one of the greatest track performances of any event. Bekele didn’t have to face the level of Tergat in the 10k and he did lose to El G in the 5k.)
Best Thon = Kipchoge (no need to explain)
Best combination of all three = Bekele