Who are essential characters

Rey, Anakin, Rg,Stormtrooper Han, Anakin

For DS squad, I would go for Droids (IG86, IG88, HK47), GS and RG.
For Jedi, Lumi (useful early and midgame, endgame, not that much, but still good to have a good healer with damage), JKA, QGJ, Aayla. Maybe IGD if in the new raid is proven useful.
Rebels: AA, Leia, STH, Lando.
Other toons: Phasma, Rey, 5s.

Remember to priorize to have a balance arena squad, then 1 or 2 backup teams for GW. Do not spread too thin, and go for only a few toons that fit together everytime.

Rule 0: If you like a character, farm them. Your personal enjoyment is the most important thing in the game.

NOW, THAT SAID... You'll get a lot of use out of Lando. In addition to simply being a very good character (AoE for days once gear starts upping his crit rating) he counts as both a Rebel (Tenacity mods, Palpatine event, Military Might event) and a Scoundrel (Crit chance mods, Credit Heist, Droid Heist) so he'll accelerate your leveling, your credits (and thus mod leveling), and get you empire shards when that event triggers.

Don't sleep on Droids, either. HK-47 might seem unexciting at first, but once its Leader ability levels up to the point where Droids start getting Turn Meter when they crit--that's when Droids really shine. The droid.team.exe is HK-47 (Arena tokens), IG-88 (Arena), IG-86 (Galactic War, Hard Modes, Cantina Nodes), Jawa Engineer (Guild), and the fifth slot varies so slot whoever you like. I've seen Poggle (Galactic War), Stormtrooper Han (Arena), Poe Dameron (Cantina), Cheif Nebbit (Arena), Royal Guard (Cantina Nodes), IG-100 (Cantina Nodes), Old Daka (Cantina), General Greivous (Shard Shop)... You name it, and someone's put them in there.

I'm pitching this mostly towards Galactic War, events, and advancing the boards because you're leveling. Arena is a whole other beast and I don't have much information on that other than what I use (Luminara, Old Daka, Qui-Gon Jinn, Royal Gaurd, Rey, Stormtrooper Han), which is good enough to finish between 100 and 50 every day so far. There was a change in the meta so now instead of finishing closer to 50, I finish closer to 100. But 825 Arena tokens a day is still 825 a day, so I'm not complaining.

You get more out of the game if you concentrate on rebels and scoundrels.
You'll get credits and training droids.
That's necessary to level up the toons you want to have.
Don't lose time and go all in on:
Ackbar, StHan, Lando, Leia, Biggs, Wedge and who else is free2play.

You'll also get a 7* Palpatine, whenever the event comes back.
After that you need 5 Jedis for Yoda
So QGJ, Anakin, Luminara and so on are necessary.
To get Gear you have to have 7* toons.
Therefor the Ewoks are strongly recommended (Teebo(!), ewok Scout, ewok elder)
The next raid is easier with Jawas/Jedis
And now, when you get everything you need the best mods.
Any 7* toon helps.
Or Jedis, Jawas, Resistance, Rebels, Scoundrels,..
So yea, that's basically the road to follow for max. effort.

The "essential" characters are the ones that let you have fun, complete events to advance, and win.