Just thinking about the best justu in naruto and am voting rasengan. Rasengan is a shape manipulating tech that is ranked A level and so far i think it is the only tech that can be upgraded. The FRS i think is the only S rank tech [probably higher] that we have seen 'naturaly possible' apart from from bloodline limit, expansion or ms. Am saying its the best because if naruto get a water element he could just combine it with is FRS and automaticaly he should get a bloodline limit [i think should be the most powerful bloodline limit because rasengan is included] he gets a 3rd element and he gets an expansion and there should be little or risk because of sage mode. WHAT DO U GUYS THINK