My vote also goes to Discipline. I'm a solo leveler, never do dungeons, and therefore I only care about things that are useful for me specifically. I tried my priest in Shadow spec (BFA content) and was struggling. Then switched to Discipline and hardly ever had any trouble again. Shields and self-heals while doing damage are amazing.
Discipline is the "tank" spec for priests. Maybe it's a bit slower when going solo, but a lot safer.