Faction: New Republic

Top Pieces to Know (non-Resistance):

General Dodonna (activation control, but currently nerfed)
Ganner Rhysode (Levitation)
Luke Skywalker, Galactic Hero (GMA to Force users)
Jedi Wing Commander + Lando Calrissian, NR Liaison (Force Dash to all Pilots)
General Wedge Antilles + Twi'lek Black Sun Vigo (GMA to followers)

There are many viable beatsticks and secondary attackers in New Republic - too many to list. But in most squads, you will need a movement breaker beyond what comes with the offensive pieces. Listed above are the main combinations to add extra movement to a squad.

Resistance Subfaction:
Resistance now has enough to warrant a separate discussion.

Top Pieces to Know (Resistance):
General Leia Organa (pure Resistance commander with +4/+10 and "slingshot" movement breaker)
Chewbacca with Bowcaster & Han Solo with Bowcaster (Accurate Retaliatory Barrage)
Rey and Kylo, Force Bonded Allies (Anarchist, monster melee piece, Spinning Blade Attack)
BB-8 (Door control, zap, defense boost to Resistance)

If you're playing Resistance, you'll probably start with one of the above options. Rey and Kylo nearly make a viable squad on their own, and you could then go pure Resistance or go another direction. If you do go pure Resistance, you probably want General Leia. The Bowcaster twins are one of the best anchors to a Resistance squad. Even if you aren't playing Resistance at all, BB-8 is playable for Manual Override, Distraction, and range 18 zap for 10dmg. Add his +4 Defense to Resistance, and he is an absolute must in any Resistance squad.

Other subfactions:
Galactic Alliance (12)
Wraith Squadron (6)
Hapans (3)

Some squads:
Professor Luke's Art Class (2013 GenCon winner)
ehh (2017 GenCon runner-up)
ReyLo (2019 GenCon runner-up)
Han & Chewie Resistance
Galactic Alliance ("Landslide Victory")

ATTA/RoS newcomers:
Luke Skywalker, Jedi Spirit brings Levitation to a pure Resistance squad.

Rey Skywalker and Ben Solo, Redeemed introduce two new offensive options. They can share each other's Force powers through Force Dyad, giving each of them many options. Ben has Han's Echo, which makes gives Resistance a quick strike option. Rey's Lightsaber Blitz lets her get up to 120dmg by spending 2 Force points, but she does need help getting into position. Fortunately, Luke Spirit, Luke Ahch-To, and General Leia are all ways to put her where she needs to be.