Which one of these numbers is not like the others?

21,15,6,16,27

16

Step-by-step explanation:

It's hard to tell what the what the problem means by "not like the others", but here's a guess.

16 is a perfect square. 16 = 4^2

None of the other numbers are perfect squares.

Also, 16 is not divisible by 3. All other numbers are divisible by 3.