It depends on which lineups you want to use, but it's roshi (imo).

Like Light1 said, you need ninjas to help roshi to be good. Same goes with 3rd raikage. It would be better if you provide us a list of your ninjas to see if any of your ninjas can work with either two.

Hi there! As other people says before me, choose which ninjas will suit your gameplay .. 3rd Raikage lineup is not a problem since pulling him with 200+ scrolls, you c4n also get some ninjas like omoi and samui that is good to pair with 3rd Raikage .. On the other hand, you now have Sailor Sakura, which is a good Healer partner for Roshi as I always see in some battles, so you only need to find 1 more ninja to join with them .. But, Roshi is versatile for me, so if i have the scrolls, i would definitely go with Roshi ..