1. Tobirama - Taught Hiruzen and Danzo.
2. Kakashi - Teaches teamwork and basic Ninja skills/techs. Also teaches how to fight effectively using shadow clones.
3. Orochimaru - May be creepy as hell and will probably want your body, but will teach you forbidden jutsu.
4. Jiraiya - Teaches Rasengan techs, basic combat skills, how to break out of Genjutsu.