MM9 - 90% challenges. What's left: Beat game without killing an enemy (just need to beat the Wily levels here), Beat game 30 times, Beat game five times in one day, Beat game killing every boss with 1 HP less (argh), Beat game without taking damage (requires skillz madder than mine at the moment).
MMXCM - In the middle of Chapter 6, just beat Shadow. Game is awesome, etc. PC notes...
Chapter 4 - X feels like the LVP here. Charge Shot twinking isn't very effective yet. Massimo has a much more useful Action Trigger in the powerful single shot of Berserk Charge, especially with Power Charge backing it. Spider of course is awesome with the two-hits-at-75-WE weapons (WE twinking is great for making 'em turn 1). Marino, when she joins, is excellent as always and is a good candidate for the second Power Charge.
Chapter 5 - Marino and Spider are obvious. X... Ice Buster is pretty nice here, but not nearly as many people are weak to ice as you'd expect for the token fire dungeon, so he's not that great. Massimo is starting to struggle due to lack of worthwhile weapon upgrades. Axl, when he joins, is a clear upgrade over either - two hits gives him the best physical, better MT than Charge Shot, and ST that is comparable to Berserk Charge. Had miinor trouble with Marino's durability here but not a big deal, LE twinking gets around it largely. Once Zero shows up, he also competes for a spot well obviously.
Chapter 6 - Where I am now. Massimo actually sees a lot of use in the first half of the chapter because of all the ranged spoiling. X... still isn't very good, as Axl does his bat-killing job better. Giving him the right buster can be handy for picking off certain blobs, I guess. Marino continues to excel (good physical, good AT), Axl gets better because his MT is called for more, possibly passing Spider even? Hard to say. Zero, of course, is pretty much a beatdown machine, just lacks variety.
EDIT: That's all for randoms. For bosses... X and Zero > Marino > Massimo > Spider and Axl > Cinnamon, or something like that.