Right now... best P1 is 4* Fully broken 10t Madara... but since that is a bit too much P2W character... I will say others, considering you have ninjas to support them...

Best offensive ninja right now is Kisame 4* BT.

Best defensive P1 right now is GNW Madara 4* BT.

For 3v3 arena, king is still Halloween Kimimaro.

For 1v1 arena, current king is probably Zetsu 4* BT, but my personal 1v1 arena champ is Izuna 4* BT..

it might be because you need either team with him (such as SS Konan / GNW Madara BT),
or you need high power to overpower your opponent xD