Consumer Reports does objective analysis of paints and stains every year. They test properties like how well it covers up what is underneath, resistance to fading, resistance to mildew, resistance to cleaners and scrubbing, etc.
Behr is usually one of their top-rated brands. Other top picks are from Kilz, Glidden, and Benjamin Moore. Which one is "best" will vary depending on the specific type of paint you are looking for (Satin, Semi-Gloss, Flat, etc). It can also change from year to year as companies reformulate their products.
You have to subscribe to see the ratings:
You can also buy a back issue.