Most useless toon in the game

Grievous is definitely top in term of abysmal return on investment, but not far behind are Geonosian Spy, URrroorrrorr (and until recently Hoth Soldier, Lobot).
Eeth Koth also deserves a mention I think.

Then all the fallen stars, toons that were once great but have returned permanently to the bench: Luminara, Droids,...

Nightsister Acolyte. She has no use, unless you want the ability to remove NEGATIVE status effects from enemies (her special removes negative status effects from all enemies in exchange for 40% TM for her on a finishing blow).

The other characters mentioned all have a purpose:

CUP can apply offense down to all enemies with his special and stun on his basic, which is useful, especially in PvE.

Eeth Koth can apply defense down or stun to droids, which is useful against the AAT.

Mob Enforcer can apply thermal detonators (useful in PvE under Boba lead) and can apply defense down.

Nighsister Acolyte is just week all around: weak damage, low health, and no useful abilities.

Fortunately it looks like her rework is imminent.

Tusken raider

Mobile enforcer... Completely useless... At least I can use my CUP in TB (Platoons)