You might get a different answer depending on who you talk to, after all it's pretty subjective fan opinion.
Am i to assume you are fairly new to Star Wars lore in general?
Here's a little guide to getting into Star Wars lore:
Right now is an excellent time to get invested in Star Wars material beyond the films.The advantage since the canon reboot is that everyone get's a fresh start.
If you have the Marvel comic app (I do!), you get access to many comics from the current canon and from the "legends" canon.
"Legends" (Formerly the expanded universe or E.U), is a collection of various media from the past, that may or may not fit with the current canon.The rule of thumb is anything branded "legends" is not to be taken like it's the word of god (so to speak), but feel free to enjoy it,since it does explore stuff not seen in the films or the shows.Sometimes the current canon will import ideas/characters etc from the legends timeline (a trope known as "canon immigration").
Other helpfull hints:
-Any media that takes place after Return of the jedi is an alternate timeline to The force awakens.
-Anything taking place before episode I the phantom menace is (in my humble opinion) fitting of the "legends" tag since it can take place thousands of years before the films.(so far there have only been some vague refferences to that timeline in the canon material).
Before this becomes a giant wall-of-text-of-doom,a great place to start would be on the small screen with:
-The clone wars (2008-2013) (availlable on dvd/itunes store or Netflix)
-Rebels (currently in season 2, plays on Disney XD, if you don't live in the USA, like me the latest episode becomes available to buy on itunes 1-week after airing).
If interested i can give you my personal list of recommendations.