Usually rabbits in fiction are heavily muscled action heroes, like Bunny Schwarzenegger and Jean-Claude van Hare. Wait, that's not right—rabbits are usually tricksters, like Brer Rabbit and Bugs Bunny. Because rabbits aren't the strongest creatures in the world, they only survive if they use their wits to outsmart their enemies. That's actually something Frith says to El-ahrairah in the legend: "Be cunning and full of tricks and your people shall never be destroyed" (6.8). This may be weird because we usually elevate straightforward types, like George Washington (who couldn't tell a lie); whereas the heroes in Watership Down would lie all the time if it would help their rabbit friends. So do the ends justify the means?
Cleverness is not the most important rabbit trait—kindness is.
Blackberry and Fiver are the cleverest and most important rabbits to the survival of Watership Down Warren, and that's that.