Pykl3, if you want the best. It's genuine level 3, with all the products, not just reflectivity. Velocity, spectrum width, VIL, echo tops etc. are all included. It has good GIS data and shows gps position, also interfaces with SpotterNetwork.
It's one of only a few apps I've bought. I'd pay 4 times what they sell it for.
But then, I'm a chaser, spotter, pilot, sailor, and all-round wx nerd. If you just want to know if you're about to get wet, any of the free apps will probably be fine. ;-)