The use of "The" should be for a SPECIFIC thing. In your example, WE are THE BEST.

If you are not using "The", that is for a BROADER array of things that are not countable. In your example, The acquaintances whom we like most are those who flatter us best.

There are NOT a specific number of acquaintances. Therefore, you cannot use "The".

Another example:
The house is red. <--- specific
All houses are red. <--- general

But if we look closely the fact of specific thing,if WE refers to a specific group of person then acquaintances also refer to a specific group of people.

Also "the" is not used only for countable things,for example "The water is cold".

Can you please comment on both the points.

still I am confused about "We are the best" Vs "The acquaintances whom we like most are those who flatter us best"

Why is there no article (the) with "best friend"? EX: They are best friends.

Rebekah wrote:Why is there no article (the) with "best friend"? EX: They are best friends.

Before answering this, I'll note that the GMAT (mercifully) does not often test the proper usage of "the", I think because native speakers seldom get it wrong. This is wonderful for teachers, as explaining "the" is a brutal task: there are so many rules, so many of which contradict one another, that it seems miraculous that anyone anywhere can use this word at all.

That said, let's clarify a few points raised here:

1:: When used before a noun (or noun phrase), "the" indicates a specific or particular thing or person. So we would say something like "Veritas is the best GMAT prep service" because there is only ONE GMAT prep service that is the best - that service is a specific thing.

1a:: When "best" is used to indicate the degree of something's greatness, however, we don't generally need to use "the": we aren't saying that we have a specific thing, only that a specific thing is best under certain conditions. "Tomatoes are best in the summer" is fine, for instance, because we're saying that tomatoes are ideal under certain conditions. (This is basically a stronger way of saying "Tomatoes are great in the summer" or "Tomatoes are wonderful in the summer".) "We are best friends" would be another example of this: "best" indicates how the degree of friendship, a stronger way of saying "We are good friends".

2:: "Those who we like most are those who flatter us best" is something similar; here, "best" is an adverb describing how well certain people flatter us, or the degree to which they do, to use our earlier justification.