Over the last couple weeks I've played X1-X5, some brief thoughts.
Mega Man X: This is the best Mega Man game, I played this one twice through just because. Been my favorite for years, no change, just great in every facet.
X2: This is X1 but worse in basically every way. It's not bad by any means but there's just not a lot of reason to play it.
X3: This is also worse than X1 in most ways, but it's at least got some weird mad ambition to it with all its weird upgrades and such, not one I'd play super often, but I'd dig it out more often than X2, it just has a stronger identity.
X4: X is a pretty eh game, the game makes it clear it doesn't care much, Zero's great fun to play as though, so he makes it the second best thus far. The plot is just intensely bad and sets a lot of precedents for the kinds of intensely bad its sequel will be.
X5: This game has, in my eyes, basically no redeeming qualities. There's some decent music I guess? It's ambitious but essentially every change it makes is bad and its story is both intensely bad and extremely verbose. They added a lot of explanations of mechanics but not any for any of the obtuse ones like the parts/boss level system. Level designs are still simplified like MMX4 but even more budget choked, Duff McWhalen's stage is terrible and the game being set up to unavoidably force anyone pursuing all items to go through it three times feels intensely spiteful.
Spiteful's really the word to describe a lot of the design here. When you get to the Sigma stages and it starts dropping in references to old MM games it's all the things that are either spiteful or tedious, quickman lasers, devil boss, rangda bangda, sniper joe pattern enemies, hallways and hallways of spikes when the game tries to make you nostalgic for old school Mega Man it seems to think the things you most want to remember are the most irritating aspects of the design.
I know the story isn't "the point" in Mega Man but special demerit to the X and Zero fight. The games implied the inevitability of this fight and the ending they decided was canon has the fight as basically a random footnote with no drama whatsoever, it just sort of happens for almost no reason. Even in the bad ending the fight needing to be followed up by both boss refights and the sigma fights drains it of any dramatic stakes. So yeah I think X5 is extremely bad. X6 will probably be worse but I'll probably dislike it less, guess we'll see, playing Xtreme 1 and 2 before I replay X6.