Itachi would be one of, if not the best mentors to have tutelage under. Kisame I see more as a mercenary trainer, than an ordinary shinobi. Both have the commitment, and overall esque for the job.
Itachi without a doubt. Kisame'd fit in ANBU, cold anc calculated but as
a teacher? Perhaps if he'd get pupils like Sasuke or Kakashi. Imagine him
with a young Naruto, Choji or Rock Lee..
Itachi would be too busy making overly complex plans to mentally scar and manipulate them to actually teach them anything. Itachi didn't trust Sasuke to grow into a strong adult by himself, that's where he went wrong remember.
Kisame would be a good ANBU head instructor. Just give him Hidden Mist pupils and they'll learn the tough way. Kisame actually has a sense of humour and can be polite, which is more than you get a lot in the 'Bloody Mist'
Itachi would be a mysterious teacher and most probably would take an Uchiha to teach, and he would probably end up killing his student...
Kisame's students sure would be killing machines but would have a moral, you know, he would teach "don't go around betraying your ally, it can be dangerous", or something like that.