I've played once with the core set and once with Twin Shadows, both as rebel, and I'll be playing imperial this time. My pals have never played IA and two of them have barely any experience with board games.

So, what's a good setup for a fun and balanced campaign? One imp player, 3 rebels. It's probably better to go with 3 heroes so they can just focus on one each, but if you wanna suggest a setup with 4 heroes go ahead and I'll consider it. Just the core set is probably best.

Heroes and imperial class are obviously most important, but you can suggest an agenda deck and side mission deck (green cards) if you have any in mind.

My suggestion would be this:

Heroes

Diala- Everbody wants to be a Jedi, and she's actually a lot of fun to play

For the other two, choose between Fenn, Mak, and Gaarkhan (remember that Diala is already melee, though- so Gaarkhan also being melee would mean you'd want to spread out quite a bit)

Finally, add in Gideon as a fourth hero that the Rebels can use- since he's heavily support anyway and can give away attacks, this should help the Rebels stay competitive with 3 players without having to play with the wonky three hero rules.

Side Missions

Well, that's up to the Rebels. Assuming you have Twin Shadows, I'd suggest R2 and 3P0's mission, because they're great to have. Rebel Sabs are also good. Luke's mission is a lot of fun, and I also enjoy the Rebel trooper mission.

Empire

My suggestion would be to play the standard Military Might deck. Tech Superiority is okay, too.

As far as Agenda sets go:

I enjoy from the Core set For the Right Price, High Value Target, and Agents of the Empire for their cards that help with threat acquisition and discounts.

+1 to the above, especially playing as 4 heroes with Gideon on shared control.

I'd just like to add - avoid the Subversive Tactics Imperial class deck. It is NOT fun for the rebels and you could turn the players off of the game if that's the first class they face. Personally I'd go for military might - waves and waves of troopers in true star wars fashion Tech superiority is a bit more trickier, you get a bunch of fancy toys but have to be careful to use them effectively.

If you're an experienced player and your Rebels not so much, I wouldn't pay too much attention to Agendas. Imo most of them are too much of a one-off bad surprise for the Rebels that are there to screw up the most cirtical part of the plan (*plays card* Oh no! That spot you just stepped in was booby-trapped, you are stunned). I pick them by selecting the side missions I'd like to play. Vader's agenda set is nice but his missions is actually very difficult to win for the Imperial. Otherwise, I liked the IG-88 missions, and if you have Twin Shadows Boba Fett is also a nice villain to show off. Imperial Industry is a good reward card that helps you balance the game by choosing how much you will use it. Royal Guard champion is a cool unit but at 15 threat you'll never get a reasonable chance to field him outside of scripted missions, just like Vader. For that matter, there's an (official) fixed version of IG-88 floating around that makes him not be a waste of points:

(It's a skirmish only card but if you're going to win him in a side mission and pay threat to deploy him, it is in principle applicable to the campaign as well. Just don't do it if he's in a reserved group for the mission and you won't upset any balance.)

I'm sorry I don't know the names of the agendas, but you can find them by looking at the rewards the missions give.