Depends on what ninja or ninjas you want...

you want a balanced team, blitz team, control team, keep in mind every ninja has a counter to it, lot of people follow the meta builds (roshis and what not) i tend more to rely on what ninjas i have fun with, so it's really up to you.

For example my case, im going for roshi, then going to lightning treasure to get Ay 3rd :)