Ichigo's tensa zangetsu, his bankai has still a lot of potential but the black katana, the pimp coat and the added speed and power alone make it a neat weapon. we saw how tensa zangetsu can
evolve after ichigo left the dangai, and that was only after 3 months of intense training. shinigiami live for a long, long time, so what'll happen to tensa zaangetsu in 20 years, in 100 years or
more? it's already a great bankai and it might be one of the strongest zanpakuto out there in a (in shiigami term) short time.
Kenpachi even without a bankai, has insane riatsu, which by far led him to many victories.. tosen was caught of gaurd, he was totally wasting time, talking to kenpachi, when he could have ended the fight in one attack lol knowing kenpachi, he doesnt care if he gets slash or stab, he gets even more fire up when this happen but nevertheless tosens bankai, by far has great ability, but acctually i dont like the design, cause its like your inside a huge tent, and your watching a live circus performance.
Tousens bankai has by far the best ability so far. And only those with good endurance would be able to deduce the weakness.. But he decided to get serious when kenpachi finally learned his weakness, but it was too late.