There's different priorities in farming and many different places to farm. What's best depends on where you're at.
First I'm going to make a list of what I consider the most universally useful toons in the game. The order isn't intended to be order of usefulness. It's a list in no particular order. It doesn't include achievement barrier and event gained characters.
Versatile Toons
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1. CT-7567 Rex. Great ship. Great on the Rancor. Great in arena. A cleanser with tm gain and removal.
2. CT-5555 Fives. Great ship. Good on the Rancor. Great in early arena. Great on the AAt.
3. Boba Fett Great ship. Great in arena. Useful for credit heists. Good in Galactic War.
4. Biggs Darklighter: decent in arena, great when paired with Wedge. Great ship. Good in all raids with Wedge.
5. Wedge Antilles: Good ship, massive damage dealer. great in arena, raids and galactic war. Best when paired with Biggs.
6. Darth Vader: Great on the Rancor, Amazing ship. Good in Arena and galactic war. Good on certain phases of the AAT (not phase 1).
7. Tie Fighter Pilot: Great ship. Useful for Potency mods. Great in a lot of early and end game arena comps. Excellent for both the Rancor and AAT raids.
8. Stormtrooper Han: Great in leveling arena. Useful for credit heists, crit chance mods, Getting Emperor palpatine, and tenacity mods. Good in raids.
9. Jedi Consular: Great ship. useful for unlocking Yoda and high level defense mods. Helps with the healer only challenge. Great low level healer.
10. Lando Calrissian: Great in leveling arena. Useful for credit heists, crit chance mods, Getting Emperor palpatine, and tenacity mods. Good in raids.
11. Qui Gon Jin: good in low level arena. Less so at higher levels until zetaed. Good on the Rancor and Phase 1 of the AAT.
12. Geonosian Soldier: Good in low level arena. Easy to gear. Great ship.
13. IG-88: great in arena up until the 80s, then merely good. Useful in credit heists, and for crit chance mods. Great on the AAT in a droid comp.
14. Jawa Engineer: great in arena up until the 80s, then merely good. Useful for crit damage mods. Great on the AAT.
15. Darth maul: great ship; great in arena. great in some raiding comps.
16. Admiral Ackbar: Fleet commander. great in galactic war, raiding, getting Emperor Palpatine, & tenacity mods.
17. B2 Super Battle Droid: Great in arena; great for galactic war; great on the AAT raid.
18. HK-47 great leader for Droids for pvo until 80s. Great on the AAT. Great for galactic war.
19. Leia Organa: Great on the AAT. great for getting Emperor Palpatine and tenacity mods. Great in low level arena. Good in end game arena.
20. IG-86: Great in low level arena; great in the AAT raid; great in galactic war.
Getting those toons would get you quite a ways in a lot of aspects of the game without having to invest in many many toons.
There are other toons that excel in one specific area of the game:
One Job Dynamos
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1. Teebo: He's great on the Rancor
2. Dengar: He's great on the Rancor
3. Grand Moff Tarkin: You need him for ship ability materials
4. Mace WIndu: You need him for ship training droids
5. Plo Koon: has an amazing ship for a full republic fleet
6. Ahsoka Tano: Has a great ship
7. Geonosian Spy: Has a ship
8. Poggle the Lesser: is good with droids.
9. Talia: Your leveling dark side healer.
10. Ugnaught: He's decent on the AAT
