Just finished 0 (for the second time) last night and, after playing the entire series (save for 1 and 2 on PS2, the Kurohyos and Kenzan!) over again since then, I honestly have to say that 0 is NOT as good as I remember it to be.
I still put it as 3rd best in the series (don't think it'll be unseated any time soon), but I found pacing issues this time around, as well as a ton of slowdown (which I NEVER had in the PS3 version, oddly) throughout the game. Add to that, the story is not as strong as I thought it was, with the beginning and mid-game being middling at best; gets superb towards the end.
People bagged to death on the bosses of 5 (will not spoil it since I know some of you haven't played it to the end) and 6, but
is truly nothing special; in fact, I found him to be fairly anticlimactic and mediocre. He does nothing until the final 3 chapters, then all of a sudden comes out being this mastermind, with intentions to take over everything, etc. IMO, he is just as random and out of the blue, as the bosses of 5 and 6. Still an awesome character though.
Thus, after platinuming 6 and replaying 0, my list of the best to worst games in the series (from my POV) is:
5 > 3 > 0 > Ishin! > 6 > Kurohyo 2: Ashura Hen > Kiwami > Kenzan! > 2 > 4 > 1 > Kurohyo: Shinso > Dead Souls
There truly isn't a bad title in the series though, the entire thing is quality title after quality title.
Anyways, I'll shut up about things, since I talk too much and looking forward to Kiwami 2 in a couple of months!