Hinata/Naruto/Kimimaro - key to a lot of hard fights and ninja exams as f2p, and are good in general.
Sasuke/Anko/Haku/Yamato - essential for 9 tails with matching main until you get rare barrier ninja.
Kabuto/Tsunade/Guy/Kurenai/Neji - good free ninjas that can still be used at high level.