This game doesn't revolve around single characters so much as team synergy, so take this list with that in mind. [Ed: also see helpful comments below]
For end game, the ability to get to 7 stars is important, so unless you are a big spender this rules out many of the chromium only characters (though reasonable chance they will be farmable at some point in time).
If you are a big spender, this list probably doesn't apply to you.
These type of lists are only a starting point and not a gospel really. But I thought it might help people out. Thanks.
Farmable, useful in several types of teams.
Darth Sidious (aka Sid; fairly fast, heal buff immunity, damage over time (DOT) synergises with Vader and IG-88)
Luminar Unduli (Lumi, great heal, leader skill useful in GW; decent special nuke with ability block)
Used in the generic, 2 healers plus 3 attacker teams these teams can take you through to the end game. Sid, Lumi plus almost any other two attackers and a healer can carry you well in Arena and GW, eg Daka, Jedi C, even Talia; plus IG-88, Biggs, Luke, Finn etc.)
A-tier unique skill
Old Daka (revive plus double stun potential; squshy in end game but will also take you through dark side battles in the main story line and useful addition to GW). Can be useful in Arena for lower powered teams to RNG a double stun to take out highr power teams, also note Phasma synergy.
Boba Fett (basically, a fairly reliable ability block which will be a battle winner for many types of arena teams)
[Ed] Poggle. He gets a special mention not just because of his obv droid synergies, but also because his buff lasts 3 turns. This means you can buff any quite hard hitting AoE character into a super hard hitting one, so IG-88 but also Darth Maul, Ventriss etc.
Droid teams (might be considered an S rank team?]
IG-88 / Poggle Lesser / HK-47 / IG-86 even possibly IG-100 MagnaGuard, for Droid teams. Theoretically Lobot might fit here too, but I've not seen him. You can throw in Chewie or Poe as a taunt protector. A bit weak on defense to many teams, but strong on offense. [Ed: @WarpSpeedyMcGee
usesthe 3 driods plus Poggle and Poe, and it's a top 3 team on offense and can hold its own defence, on my server]
Captain Phasma (almost unique, really shines in counter teams as the counter attack can also call an assist); Counter teams; Kylo Ren, CT-5555 Fives plus any counter unique characters. Counter teams seem to played adequately well by the AI; and a well timed Phasma on the offense is a winner.
Sid as lead usually and then high crit attackers. Some of these attackers have been nerfed in the recent update (eg Resistance Pilot, Trooper) and some are amazing at high stars eg FOTP, First Order TIE Pilot, but he's very hard to star without much chromium speding; Leia, Lando (a high star Lando, can pretty much wipe you turn 2 as his skill resets)
These are A teams if mixed with Counter (and you willl likely have Fives as core, see above) but without Counter, they are a bit weak to Sid in my view, though AI seems to do OK with them.
You can build strong teams around many of the chromium unqiue characters eg Lando, Old Ben, Leia; Darth Maul (plus eg Poggle) but unless you can get them to at least 6 stars they will fade vs a 7 star 2 healer plus 3 attacker; or a 7 star whale team.
There might be undiscovers gems too that synergise, eg Ackbar ? Underworld Police ? but the above are probably the "safest" teams.