I've kind of got my wires crossed over which one to use.
There's of course the construction - " of all people/ places etc" (Casablanca springs to mind right away, where the protagonist says something along the lines of " of all the gin joints of all towns [...] he walks into mine ")
And then there's on the other hand this one: 3 out of 10 cases, 5 out of every 6 people etc.
I'm still having a hard time putting my finger on the right way to say this though:
Why would they choose us, of the thousands of people who probably send them letters each and every month? / ...... OUT OF the thousands of people who....?