It depends on what you want to do with it exactly, but if you're talking in an all around sense find one that feels comfortable and easy to control/ light enough & thin. Secondly, one that holds an edge is a good idea or you'll spend more time at the grinder than bow making.
I rarely use a hatchet aside from splitting staves. My favorite chopping tool for shaping is an old meat cleaver. I get a lot more control out of it. There's also a lot to be said for a good draw knife. I bought a vintage one on ebay for half the cost of a new one and I guarantee the steel is twice as good as any thing made today. It still had a factory edge when I got it. I had to hunt for it though there were a lot of them that had seen better days.
If you're buying new read the reviews online for that brand if it's junk someone will have pointed it out.