Question: I'm normally pretty good with grammar, but this one just stumped me. I was trying to write something similar to "you're one of the only people who [is/are] nice to me", but I'm not sure if I should use "is" or "are".

Which sentence is correct, and why?

> 
> You're one of the only people who is nice to me.
> 
> 
> You're one of the only people who are nice to me.
> 
> 
>

Answer: 
To determine agreement, you need to find the head noun being modified by the relative clause *who is/are nice to me*. First let's find the **head noun** candidates . . .

> 
> You're one of the only [**people**] [who are nice to me].
> 
> 
> You're [**one**] of the only people [who is nice to me].
> 
> 
> 

Now let's test them . . .

> 
> You're one of the only **people who are nice to me**. --> Of the
>  only **people who are nice to me**, you're one.
> 
> 
> You're **one** of the only people **who is nice to me**. --> Of the only
>  people, you're **one who is nice to me**.\* (incorrect—which only people?)
> 
> 
> 

Sometimes you do have a choice! Here are some more **head noun** candidates . . .

> 
> Gruyère is one of the Swiss [**cheeses**] [that are used in fondue].
> 
> 
> Gruyère is [**one**] of the Swiss cheeses [that is used in fondue].
> 
> 
> 

Again, let's test them . . .

> 
> Gruyère is one of the Swiss **cheeses that are used in fondue**. -->
>  Of the Swiss **cheeses that are used in fondue**, Gruyère is one.
> 
> 
> Gruyère is **one** of the Swiss cheeses **that is used in fondue**.
>  --> Of the Swiss cheeses, Gruyère is **one that is used in fondue**.
> 
> 
> 

Deciding whether something (e.g. *one*) is a pre-modifying quantifier or a head noun can be tricky. I imagine any rule that might exist would be way more complicated than applying the tests. Meanwhile, you can always write your way out of the situation . . .

> 
> You're among the few people who are nice to me.
> 
> 
> 

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Further reading:

[Quantity Phrase Nouns—Agreement with Verb in Relative Clause](https://www.grammar-quizzes.com/agree4b.html#quantityphrase)
The sentence makes no sense as it stands. Perhaps you mean something like this?

You're one of few people who are nice to me.

You're one of only a few people who are nice to me.

It's not clear what you mean, but at least your words "one of ... people" shows that you are speaking of the other person as one of some plural group of people who are nice. As Mohsin Kamal says, this means that the verb should be "are". But "only" makes no sense. Only what? "Only a few", perhaps? "Only" and then some number, perhaps?