Because I'm the editor of Engadget by day and a volunteer coach in my free time, I often get asked which GPS watch or fitness tracker to buy. (People also ask what I'm wearing and the answer is: All of them. I am testing all of them.) For my part, the best running watches are quick to lock in a GPS signal, offer accurate distance and pace tracking, last a long time on a charge, are comfortable to wear and easy to use.
Advanced tracking features like monitoring VO2 Max, or maximum oxygen intake during workouts with increasing intensity, are also nice to have, along with training assessments to keep your workload in check and make sure you're getting in effective aerobic and anaerobic workouts. It's also a plus when a watch supports other sports, like cycling and swimming, which all of these do to varying extents. As for features like smartphone notifications and NFC payments, they’re not necessary for most people, especially considering they drive up the asking price.