Which synergy is the strongest right now?

Each synergy has its pros and cons. FO looks good right now, and looks like it will have some staying power as the meta changes. FO and Resistance will probably get some love as ep 8 comes out. As F2P, the FOTP is hard to farm, though.

Overal I'd prob say first order, although a Nihilus lead may be a counter to them as their Crit immunity won't mean much, Nihilus dispelling their advantage and stormtrooper's taunt

You're a pleb Mike

Chaze /thread

Depends on what you mean under 'strong synergy'. The strongest synergy, to my mind, is Droid's one with HK-47 plus JE, since they became a powerful killers due to it and are absolute garbage as separate units, while many other synergies, including Rebels, are made of toons who can work in mixed squads and still be viable.
If you meant which faction is working the best together, I'd say it is Siths atm. Maul(l), Sith Assassin, DN. They are extremely fast, pretty survivable and have a very useful kit: daze, cooldown reduction (improved version of ability block), instakill.
Resistance are very easy on defence. Their 'engine' is working while they can expose. Fast toon with mass tenacity up buff (Yoda/Rex) and whole their beauty is wiped out. FO right now doesn't have their own mass dispeller and mass cleanser. It's a problem. But since there will definitely be some new toons released before the new movie in 5-6 months, both Resistance and FO can get some toons making them shine. In other words, their factions are very promising.
Jedi are pretty universal. My only zeta is on QGJ and I love it. Clones are pretty powerful. I'm slowly working on them right now. Empire meta has partially gone and now only some mixed sith+empire teams are really viable. Scoundrels... I've never met fearsome ones. Ewoks, Geonosians, Tuskens and Jawa, it's not serious.
Soon, I believe, we will have a relatively new faction which will get some strong toons, namely separatists. And there might be a renaissance of NS with the release of Mother Talzin. Btw, droids still don't have their zeta.

Maul Zeta lead is the meta with the crazy sith synergy going on right now. In a December 2015 shard, everybody is switching to sith

I like B2, Vader and Darth Nihilus.

B2 blocks positive effects and cleanses making Vaders Force Crush stick. Then Nihilus does his Drain Force speeding up annihilate by 5. This is guaranteed knockout, once or twice per match. That doesn't even include Vader's knockout with Culling Blade or the other 2 toons you have on your squad.