Its "least"
Little-less-least.
little, littler, littlest little, less, least
littler, littlest less, least
littler, littlest or less, least
positive degree=few comparative degree=fewer superlative degree=fewest
Sum is a noun and a verb and, as such, does not have a superlative degree.
"Less" is the comparative degree of little. The superlative degree is least.
little, littler, littlest little, less, least
littler, littlest less, least
littler, littlest or less, least
the superlative degree of much is most
No, the superlative degree is "most tirelessly."
The superlative degree of "old" is "oldest."
superlative
The superlative degree of stout is stoutest.
The superlative degree of "broad" is "broadest."
The superlative degree of heavy is "heaviest."
The superlative degree of little is littlest. Otherwise it would be most little, and that's wrong. Superlative means expressing the highest degree of a quality in grammar.