Doomtrain is imo the best GF since, if I remember correctly, he gets vitality 0 status effect on Omega Weapon!!! I wouldnt say Eden or any of the others are better simply b/c aura + limits are better dmg than any Gf any day. Diablos was pretty good for a while too since I always beat him the second I got the lamp, and he has a could good abilities and junctions if I remember right. Also, why would you not get every GF possible since by equipping more to each character it allows you to junction more for them as well as open your party up to more ability combinations?