I'd say that X8 was well beyond X5, X3, and Xtreme1 in the quality department, all of which predate the games you mentioned.
X5 was no quality title, either. It was, like X7, great ideas with bad execution. Not *AS* bad execution, but still. The power-ups were tied together in senseless ways, the ties between power-ups, ranking, and boss level shouldn't have even existed, the game was made considerably more linear with an obvious "intended" order of Grizzly to Red, the Gaia Armor is impossible to obtain reasonably early (you must visit every stage and you must have actually cleared Grizzly Slash, Duff McWhalen, and Izzy Glow), and several tanks are impossible or near-impossible to reach as Zero. It also has quite possibly the single worst boss AI I have ever seen in the versus-Zero battle. And whoever came up with the idea of Zero dying in a boss explosion, which is in of itself wrong enough, but also without regard to how close he actually is to said explosion, needs to be shot.
X6, despite rushed development, fixed quite a few of those flaws. The power-up system was opened up, allowing all hell to break loose on your end, and Nightmare Zero provided a PROPER PS1-era Zero battle (without adding a single sprite, I might add).
X8 simply suffers some presentation issues due to crapass art style and generally lazy visuals inferior to Command Mission. And even if most of X's special weapons were pointless, it did give us some cool stuff. The armor mix-and-match (even if presented lazily) is long-overdue, in and of itself adding a lot of value to the game. We got a lot of stuff back that X7 forgot (multiple armors, secret forms, and such). We got the first-ever secret characters (even if clones, it's still cool). We got the best-ever Ride Armor (INFINITE GROUND DASH AT LAST!!!) and the first-ever Ride Armor theme music, which kicked ass.
quality drought that is X6
*gives Soultrigger ZEE UPPERCUT!!*
Barring Sentsuizan and the annoying Nightmare effects, X6 does not lack quality. It's just challenging in a manner that is untraditional for the franchise. Excellent service to people who know the formula like the back of their hand, I say. The game allowed far more freedom, customization, and power-up potential than any other in the series (up to 16 Life Ups, 8 Energy Ups, and 5 Power-Up Parts all stackable on either character and any armor), and unlike the Zero series, it gives you insane enough enemy mobs to provide an engaging experience even after you've massed all that power (which incidentally you can obtain without the use of a single special weapon; therefore allowing you to earn for yourself the equivalent of the hacked passwords of the SNES days). No other MegaMan title gives you the sense of a large-scale battle that X6 does.
I've often said, X6 is to MegaMan what Lost Levels is to Mario. It's not high production value and it's not newbie-friendly. But it throws the kitchen sink at you and makes a great challenge for people who know the series inside-out.