if you dont have 4* Gengetsu get him coz he have strong BT.

if you have him and want another BT ninja get Danzo and wait for 2nd firework to get him 4*

or if you need some ninja that have strong control take Ibiki

or if you want some fun ninja get Raikage or Sasori depends on who you like more

over all its up to you. Gengetsu is strongest and easiest to BT and get more frags, Ibiki is 2nd strongest