I gotta agree with Flame that X5 blew as a possible series ender. Such an obvious cop-out as a boss explosion death after the baddie is already beaten leaves a lousy taste in one's mouth. About the only thing X5 did right, story-wise, was referencing Iris and giving us a glimpse at the kickass viral-infected Zero in the alternate storyline.
And do not get me started on the gameplay flaws. The title was a lot of experimentation and very little polish. Ironic that X6, rushed as it was, corrected most of it.
Well, aside from the terrible nightmare bs that I've never thought was good, after replaying X4/5, my problems with X6 are nearly all gameplay issues. The levels are just really really frustrating and meh. So thats the direction I dislike, there. Again, aside from the Nightmare stuff, I really liked that it followed straight up from X5 instead of ignoring it, even though X5 should've ended it.
However X6 remains the only game in which you can (without password-hacking, anyway) obtain everything without beating anyone. It's a lot of fun and it makes replays a blast, not to mention you can stall Nightmare Zero forever with the ol' Shadow Armor/Ultimate Buster combo. One of the few things X7 did right was introduce New Game Plus, so the mechanic could be preserved without the ingeniously challenging level design (I mean the CORE level design, not the nightmare crap).
While I disagree on X8 being almost perfect, we can all agree on X7
Even X7 got a few things right. Not many, but a few. For one thing it's the only game that made decent use of a Jedi-esque reflect saber. Z4 tried, but the fact that they virtually eliminated standard projectiles made it worthless.