the game tells you what it thinks the best moods are for each ninja, also you can look at the ninjas growths and see which stat will get the most out of a stat buff

The Inn game stats are funky, one of the ninja days life and attack, but he had no taijutsu moves. I don't trust that completely

It all depends on how you want to use said ninja(s) in a certain team.

The best way to decide which mood to increase is to look at the growth rates and decide from there.

I also believe that, when finding your mood, you should think about what you're using that ninja for in most, if not all situations. for example, Jigokudo heals, resurrects, and does a chase with hell king, and you see he's already decent in his offensive skills. you probably want him to either heal more or give resurrected ninjas more life, which seems to be the case most of the times you use him in a team. so you give him a boost in resistance or ninjutsu to benefit those. now i'm not looking at it right now, but i think it says his important traits are Life and Ninjutsu. it's not that important you need to use those 2, in fact i think of that as a default option for it; like a defensive and offensive trait for him. And you could get more power off one mood than another, so make your decision a wise one before you start spending on them.