I don't know if it's best since it's the only one I've ever really used, but I use The Weather Channel because it gives me hourly percent chance of rain. I'm a college student so I have to walk a lot during the day and in the SF Bay area, it can be sunny one minute and raining the next, sometimes both at the same time.
I guess my only comparison is the Sense 3.0 weather app, and while it's all nice and pretty, it doesn't have hourly % chance of rain, therefore I can't rely on it.
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