I like Onmund because I appreciate a Nord who has finally left behind the social expectations of being a 'strong, hardy man', and has instead persued the teachings of the Arcane Arts.
I also like J'zargo too simply because he is quite eccentric and humorous. For instance, he is heavily implied to be a thief during the excursion to Sarthaal. I remember he says something along the lines of 'hopefully, J'zargo will find some things here that are small and can fit in my pocket. Things that no one will notice if they went missing.' Additionally, I love how confident/arrogant he is, as well as his jealousy towards the Dragonborn, as such when he tries to have him killed in his side quest of the tinkered Flame Cloak spells.
J'zargo is the best for level cap, but he's also shoot first-ask questions later...my preference is Brelyna because she waits for me to do the attacking or get attacked...and she is all business when the time comes...she also charms me when I talk to her, trading items with her "I'm a mage not a pawnbroker, you know"...and told to wait..."I'll wait here, just don't forget about me"...Brelyna with the right gear has sent Forsworn, and hags/mages running before I even knew the fight was started