Ragnos may well be the most consequential of all Ghosts - certainly of all the Sith spirits. Sadow and Nadd are next in line for training the next generation of Dark Lords. XoXaan is pretty consequential since she trained Krayt, whilst Muur, despite his longevity, isn't very consequential.
For the Jedi, it might be Qui-Gon Jinn, actually. He taught Yoda and Obi-Wan how to become Force spirits.
I forgot about Surik. Without her, Revan would've been destroyed by Vitiate and the Dread Masters and the Republic would've been attacked and destroyed ahead of the Great Galactic War. There'd be no Treaty of Coruscant, either.
Qel-Droma's Ghost teaching Drain Resistance to Anakin kind of saved the Republic too, yeah.