Greece - Plato's Republic: 10 (13-3) It's boring. Yea, it's good, and it's flexible, but heck if it's not boring.
Maori - Mana: 18 (17+1) But Mana is a rather good ability, and is also flexible in victory type (though not too flexible in map type, as it does better on island-based maps than on continents/pangaea), AND it's incredibly fun! It's perhaps the most game-changing of any ability in Civ 6. You start the game in the middle of the ocean. It's great for exploration, and exploring the world first gets you lots of goodies, like free envoys, era score, and tribal villages. Bonus production on unimproved tiles so that you get incentive to keep your forested lands untouched, and you can't harvest resources, which is a disadvantage but is thematic and can change the way you see the map, especially combined with the other things. Plus, your fishing boats not only trigger a culture bomb, which I always like, but also get +1 Food, which can be pretty juicy on an islands map where you will be the one to find all those nice rich coastal lands first. Overall, the disparity between performance on different map types and the presence of downsides could make one argue that this ability isn't the best, but it's the most unique of them all and is often quite powerful as well.
Maori - Mana: 18 (17+1) But Mana is a rather good ability, and is also flexible in victory type (though not too flexible in map type, as it does better on island-based maps than on continents/pangaea), AND it's incredibly fun! It's perhaps the most game-changing of any ability in Civ 6. You start the game in the middle of the ocean. It's great for exploration, and exploring the world first gets you lots of goodies, like free envoys, era score, and tribal villages. Bonus production on unimproved tiles so that you get incentive to keep your forested lands untouched, and you can't harvest resources, which is a disadvantage but is thematic and can change the way you see the map, especially combined with the other things. Plus, your fishing boats not only trigger a culture bomb, which I always like, but also get +1 Food, which can be pretty juicy on an islands map where you will be the one to find all those nice rich coastal lands first. Overall, the disparity between performance on different map types and the presence of downsides could make one argue that this ability isn't the best, but it's the most unique of them all and is often quite powerful as well.