The strongest RPC is presently Kirugetsu, and for good reason. He's a 44 year old war vet, older than Sai, and was a key player in the resistance.
Other notable characters would of course be the Nara and Shira. Nara are almost universally powerful due to their insanely cheap moveset. Shira herself is useless if disarmed, carries a blade I firmly believe she can barely swing, requires combos for anything and cannot fight for more than a few consecutive minutes. I presently believe that 4/5 of the other Genin could probably beat her with simple counter-kenjutsu tactics. She also has absolutely no resistance to genjutsu and no ninjutsu, so she's basically Rock Lee with a sword, except that she can't even be trained for the Eight Gates unless she forfeits her sword for a while.
Even her Kekkei Genkai is half-useless in her present state. The ability to predict attacks is utterly useless if, let's say the Maruki, is attacking her. He'll be able to see her counter-attack just as quickly as she sees his attack, and his Juken is more advanced than her taijutsu. If she can't swing that big ass sword, she's totally fucked. Same concept applies to Samurai. If ninja can't make handseals, they can't perform ninjutsu, which is why taijutsu experts are the biggest thread to Samurai.