I still think that, in their current state, Eureka Relics are the best. Every Fate you do, you progress in your quest, light farm goes very fast with Louhi and Cassie spawn mobs, and you can get one in around 20 hours of farm. Maybe Hydatos might recieve a small boost in term of crystal drops, but overall, it's the most consistent experience. It might have been awful when it released, but in this current state, it's nice. A relic takes around 15 hours to do, which is long enough to be rewarding, but not too long.

ARR and HW had their awful steps that took far too long (the light step is only ok since you can do it unsync, it was wild before that)