The plural version is more common according to Google ngram. It is also used as an example in Cambridge's dictionary.
However, I would like to know whether the plural and the singular versions are equally perfect to use without sounding unnatural.
If there is any difference, of course I would like to learn about it.
Singular:
It's/I'm acting in your best interest
Plural:
It's/I'm acting in your best interests
Google ngram is provided for both with "your" and without:
your best interest,your best interests,best interest,best interests
Best Answer
From the definition of the word interest in Macmillan's Dictionary
Sense 5
[countable/uncountable] an advantage or benefit to someone or something
be in your (own) interest(s) (=bring an advantage or benefit):
have someone's (best) interests at heart (=want to help them):
be in someone's/something's (best) interest(s) (=bring an advantage or benefit):
However, The FreeDictionary gives two versions for the idiom "... at heart"
have someone's best interest(s) at heart
to make decisions based on someone's best interests.
And from Oxford Dictionaries, it seems that interests is generally more common.
in the interests (or interest) of something
2 For the benefit of:
It seems it is a question that may bug some native speakers too!
A relevant question on the Web: question for grammar junkies.....
NB: Since the proof was in Macmillan's dictionary already, I should consult more than one dictionary from now on. In fact, many times I find clearer answers and definitely some examples when they are not provided in Cambridge's. I like Cambridge's look and feel more and it is a decent work to be honest.
Edit: This one is even more interesting!
in the interest v. in the interests
Edit 2
With this, the post should come to an end
Best interest or interests? MIND OUR ENGLISH
The Star Online , Thursday April 10, 2008