to the lowest degree vs least

to the lowest degree is not a noun while least is a noun.

Word	Noun	Adjective	Verb	Adverb
to the lowest degree	No	No	No	Yes
least	Yes	Yes	No	Yes

As adverbs, to the lowest degree and least are synonyms defined as:

to the lowest degree and least: used to form the superlative, smallest in size or degree

to the lowest degree (noun)	least (noun)
	something that is of no importance

to the lowest degree (adjective)	least (adjective)
	the superlative of `little' that can be used with mass nouns and is usually preceded by `the'; a quantifier meaning smallest in amount or extent or degree

to the lowest degree (adverb)	least (adverb)
used to form the superlative, smallest in size or degree	used to form the superlative, smallest in size or degree