If I were to rank the disciplines:
1) Telepathy (Invisibility, Shrouding, and Psychic Shriek are all great; Hallucination is terrible, others mediocre and situational)
2) Divination (Prescience is probably the most useful Primaris and many of the other powers are handy. Nothing overwhelming, but always pretty useful except Scrier's Gaze)
3) Malefic Daemonology (The various summoning powers are very handy, Cursed Earth is fabulous if you play Daemons)
4) Biomancy (Smite is pretty good, Endurance and the toughness-knock are great, Iron Arm is awesome)
5) Sanctic Daemonology (good powers, but only Grey Knights can use it without their heads exploding, I think. Never see this one taken)
6) Psychokinesis (or whatever it's called. So-so powers, nothing that amazing. Not actively bad, mind you, but still)
7) Pyromancy (unless you face hordes, it kinda sucks. Lots of fairly lame witchfire powers)
As for the specialty disciplines, the Runes of Battle is freaking amazing, Runes of Fate is varied and generally useful (Guide is a great Primaris), the CSM lores all suck to varying degrees (Tzeentch especially, but Nurgle and Slaanesh aren't any prizes. They're so-so, I guess). The new Lore of Change is very good right now (Prismatic Gaze? Boon if Flame? Yes please!), the Nurgle and Slaanesh Daemon lores are okay. Don't know enough about the Tyranids or Orks to say much.