For me its sharingan since you would be able to predict where which tenketsu point that any Hyuuga will try to hit......and i think tht Kimimaro's kekkei genkai is good as well but there are alot more we haven't seen in Naruto so far we only know 2 from Konoha,1 from mist,1 from Kimimaro's family....thats about it =/