I wasn't going to bring up the Turn A, since a lot of fans(UC and AC) abhor it. I've found the first 18 episodes to be very well made, if a bit slow to get going, with both sides falling over themselves to prevent the war from esaclating to a point where it can't be stopped. But, yes, the Moonlight Butterfly from either the Turn A or the Turn X is the single most powerful attack that can be conceived of. And it must fall into UC continuity because all the suits they are digging up are Zakus and Kapools. But then, there's the vision of Wing Zero later in the series...
Deathscythe would fare well against other Gundams, it has a very effective stealth system(not as effective as Mirage Colloid though) that he can attack through, he's very fast, and there's always the psychological aspect of the design since Deathscythe was designed to frighten the enemy with it's demonic design.