Lands like Gaea's Cradle and Evendo, Waking Haven also let you create an amount of mana equal to the number of creatures you control, which will really speed up your squirrel production
Endurance is the most castable of the cycle, which is part of its strength
It does, however, have some fierce competition, Burgeoning, for instance, offers a similar strong land-playing ability that even beats Exploration in Commander
This is more than a Commander card
But rather than losing an increasing amount of relevance every turn, the Theros staple can dodge an Azorius Charm and hold off an Alms Beast
Biomancer's Familiar This is certainly powerful enough that you may wonder why it's not higher on the list, but I think as powerful as it is, it's very narrow