A common expression to that one:
You are unquestionably one of the greatest[b] friends one could wish for.[/b]
It works with the word in plural form:
They are, without a doubt, one of the greatest friends one could wish for.
You are, without a doubt, some of the greatest friends one could wish for.
Note that to avoid ambiguity, I changed "one" into "some" in the second sentence.
Being a bit more dramatic about that, you could say "you are the best friend one could have./you are the best friend by a long shot, etc".
Plus "you are the ultimate friend (one could have)!" (then, if want you elaborate more on the "why is that so.")