What metal is most suitable for a ladder submerged in an underground water tank?
Stainless steel is the obvious choice. Which exact grade is probably more dependent on what you can easily find than what the "most perfect for the job" grade might be,  but 316 or 316L would be high on the list and are quite commonly available.

Be very careful about ventilating the tank if entering it "for maintenance" - tanks can be deadly. If the water is clean going in and there's no route for contaminants to enter, the need for internal maintenance should be minimal.