Standard Key to Test Harmonicas

Hi, David. Hope all is well.

I mostly use Sp20s in G, A, and C that I bought at a real good price in a set. I also have a Marine Band in C and a Manji in A. I am curious about trying out other brands, models and keys but really also want to compare apples to apples. My question is premised on the key that will be most used by blues players. I assume that is A, but perhaps I'm wrong. 

So, my question is this: What is the best key to buy in order to compare different harps?