"Greater part" versus "Best part" describing duration?

Question

"The delegations from Beauxbatons and Durmstrang will be arriving in October and remaining with us for the greater part of this year. I know that you will all extend every courtesy to our foreign guests while they are with us, and will give your whole-hearted support to the Hogwarts champion when he or she is selected.

— Harry Potter

Does the meaning of greater part of a duration the same as best part of a duration? which both means the most part of a duration? Will the meaning change if the above quote is changed into "best part of this year"?

Answer 1

The "greater part" of the year is the larger part; in other words, more than half.

The "best part" of the year can mean the larger part, but it can also mean the most favourable part.

If it's important to avoid that ambiguity, use "greater part" for the larger part and "best time" for the most favourable time ("Autumn is the best time of the year").

Answer 2

Will the meaning change if the above quote is changed into "best part of this year"?

No. The meaning will stay the same.

However, the best part of the year can also mean the part of the year I like most, as in

1 June is the best part of the year. I like it the most.
2 Summer is the best part of the year. I like it the most.
3 September through May is the best part of the year. I like it the most, because that is when school is in session and I love school.

Note that Greater part of the year does not mean the part of the year I like most. So, you will change the meaning of the Sentence 3 if you use the greater part of the year. With greater part it will mean longer part or the more long part; and it can be equivalent to the most long part, depending on the length of the duration you are talking about.

Answer 3

In my decades of experience being an American, Americans rarely (if ever) use "the best part of the year" to mean anything but "the part of the year I like best." (So if you say "he stayed for the best part of the year," your listeners will almost always expect you to tell them what you consider the best part of the year to be, such as summer, winter, spring, or autumn.) Personally, I would say you should never use "the best part of [a duration]" when speaking to Americans, unless you mean the part you like best. Some might understand what you mean, but I think most would not.

Americans may say "the better part of the year" and usually mean "the larger half of a year." Saying "the greater part of the year" or "the larger part of the year" or "most of the year" would also be understood, without the ambiguity of using "better" (which could also mean better-versus-worse).