Jackal (coming around again soon) is the closest thing to a must have. Grab two or three of him as wars are 70% purple tanks now. He is also a key titan hero. He is the only 4* I can list as yellow is really lacking anything else special in the 4* area.
Vivica with costume or White Rabbit are must haves for the best titan scores (along with Jackal).
Guinevere is “nice” to have, but the old girl has hit the wall.
Joon and Poseidon are solid snipers and Drake Fong is a favorite on defense.