If I could only afford one- and planned to ride in high-traffic areas- I would buy a hi-viz green or yellow for safety.
After that... I give a lot of consideration quality of the fabric and fit. I don't like them skin tight, but I don't like the really relaxed fit either. I have a couple of jerseys that are really good for hot weather (I live in the desert!) like one with a mesh back.
I look at color - no black for summer riding, and avoid colors that look awful on me. I am partial to blue, because it's my favorite color and matches both my old and new bikes. I bought a couple of really inexpensive jerseys at first, but don't like the feel and fit. My current favorites are made by Terry, Twin Six and Pearl Izumi.
And last but not least- given all those things, I'm a serious bargain hunter and look for close-outs and sales.
2016 Specialized Ruby Comp disc - Ruby Expert ti 155
2010 Surly Long Haul Trucker - Jett 143