Who are essential characters

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.