X6 is definetely where the MMX series jumped the shark, it can be fun but the fact that the final levels are borderline IMPOSSIBLE to beat and force you to look everywhere for the right armors makes this borderline mediocre.
X7 is undeniably the worst game in the entire series, there was NOTHING good about it. The graphics were terrible, the dubbing was even WORSE than X4's, unacceptably long loading times, the controls were awful, gameplay was frustrating and atrocious, it was just a terrible game.
I haven't played X8, but it looks a lot better and more fun than X7.
I haven't played command mission and i don't plan on getting it anytime soon.
Oh, and Xtreme And Xtreme 2 were okay but they can't compete with the main series games.
Maverick Hunter X looks like a good remake, but i don't plan on getting a PSP anytime soon so i might never get to try this one out...
I'm not sure which I would say is the best... Defiantly one of the first three X games, maybe X4. X6 is terribly underrated, but defiantly not as good as the other games.
If I had to choose between the first six X games, maybe X2 (since I basically grew up with that one)... To me, it's just such a fun game!
the worst would have to be X5. It just seemed so silly on so many levels, with the whole "Countdown" thing, the fact that you can skip to the fortress levels as soon as you get done with the intro stage, and the game gives you no motivation to actually want to get the upgraded armor, since it gives you that armor from X4 on the get-go. That's one thing I liked about X6, that it gives you the motivation to get the armor (unless you use Zero), plus it's difficulty is something I welcomed. Plus, X5 introduced all those anime characters, and you couldn't skip the Alia things... in X6, you could (mostly) Completely ignore her.
X6 would be higher on my list... if it wasn't for Nightmare King/Nightmare Mother........ Other than that, after you get used to the game, it really isn't hard.
If we were going past the first six games, then X7 would be the worst. X8 is underrated, as it's actually pretty fun (short, but fun... plus, you could turn off the navigator all together!)
X2 X1 X3 X4 X6 X5 --- Revenge is a dish best served with Pinto Beans and Muffins!