There are no big surprises.
Individuals sports like golf and tennis are going to have a greater spread of champions. There are very few Tigers. The Masters is the most competitive competition in the graphic. Tiger's first title in 1997 is not included. His 2019 triumph is included. The US Open Tennis tournaments are also at the more competitive end of the scale. The women's tournament being more competitive than the men's tournament.
US team sports are more competitive than the UEFA Champions League. A variety of potential explanations could contribute to this feature - including the relatively closed nature of US Major Leagues, the draft system, and the higher scoring in US sports.
US team sports and the Champions League have a league element (regular season) followed by an elimination tournament element (post-season). On balance, the elimination tournament should give "weaker" teams a better chance. The two tournaments with only a round-robin structure are at the less competitive end of the spectrum. Teams play each other once in the Six-Nations rugby union competition. The Premier League is a Home & Away format.
The teams in the Six-Nations are representative teams with no trades/transfers. The teams and players represent their countries. Path-dependency or tradition can play a large role in these competitions. The Irish team sports of hurling and gaelic football are also played by representative teams (counties rather than countries). Kilkenny have won 55% of the hurling titles. The last five gaelic football titles (25%) have been won by Dublin.