.

Likewise, is most favorite a word?

Favorite is literally a Romance variant of 'favored' or 'favored one'. It should be able to describe comparative levels of favor and used to do so. The OED lists a cite for 'favoritest', presumably for the word's two-syllable pronunciation, but 'most favorite' was always more common.

Additionally, is most favorite redundant? Logically it's redundant, since favourite means most favoured of all things of a certain type or class. But as you say, it is used so often that it's correctness becomes almost immaterial. This also applies to phrases like most perfect and most complete.

Herein, is most best grammatically correct?

Bestest means “the very best,” “the best of the best,” or “better than the best.” It is formed by adding the superlative suffix -est to the word best. This practice is grammatically incorrect, because best is already the superlative of good.

Is Least favorite grammatically correct?

If you're asking if they both mean the same thing, then no, not really. “Least favourite” is a far weaker statement than “most hated”. Hating something or someone is a more powerful feeling than disliking it/ them.