I have a ton of katanas for all sorts of different situations, here is a list of a few:
Inside abyssea I use Kamome for really anything aside from nukes.
Outside of abyssea I stick with Oirandori for the most part.
For nukes inside or outside of abyssea I use Sekka+2x2(MAB/INT)
For PvP vs PLDs or RDMs I single hand a Tsukumo with add effect silence if they aren't using ice spikes. If they keep ice spikes up most of the time I just put on a Terra staff, or use the MaB/INT Sekka+2s.
For tough solos on things that have very high acc/low delay(Dark Ixion 100% Acc w/ shadows up) I use Hochomasamune. /RDM, 26% Haste, Fast cast gear, and the utsu necklace; Ichi's recast is only 16 seconds, and finishes casting at around 40%. That paired up with AF3+2 boots, and Ni can make keeping shadows up a joke even on the hardest of things.
And lastly the only time I've really found a use for my Enchu +2 is when I was soloing Fafnir/Nidhogg. At the last 20% Oirandori just can't keep up with the TP you need to spam waltz. Even Enchu +2 cuts it pretty close.
For the most part a NIN really only needs a Kamome to offhand with a Kannagi, Oirandori probably pulls ahead of Kamome outside of abyssea. However I'm sure Kamome would be fine. I know a lot of the stuff I have is a little... over the top, but it's what I've found to be the best in each situation so far.