Active Sky all the way.
Active Sky 16 (which is the most current version) is probably the best weather engine right now, but the previous version named "Active Sky Next" is still a very excellent product (and cheaper, because often in sales).
I have Active Sky Next in P3D v3.3.5. It allows me not only to get the real weather, but also the real weather of the date I chose for my flight.
For example, if today I start a flight in the sim, and I choose Seattle Tacoma on the 1st of August 2016, then the weather I get in the sim will be the weather they had in Seattle on the 1st of August. That's the historical weather mode, and it's also available in Active Sky 16.
Active Sky 16 can coupled with another product, ASCA (Active Sky Cloud Art). ASCA allows Active Sky to use different cloud textures based on the type of weather (instead of using just one set of clouds like normal weather engines do).
However, this raised the price a bit too much for my taste.
Also, I'm so satisfied with Active Sky Next that I don't feel the need to switch to the next version yet.