I mean, you don't necessarily need a speed BOOSTING nature, but definitely don't take a nature that lowers it. Mewtwo's just not bulky enough to warrant that.
And really, only Mewtwo Y can truly get away with not having a speed boosting nature, because of the additional speed that particular mega evolution gives.
EDIT: That being said, Mewtwo X's Fighting secondary type is really the best answer to stuff like Darkrai. You have enough bulk to sponge a neutral Dark attack if need be, and more than enough force (especially with STAB) to utterly shut it down.
Mewtwo Y can probably also deal with him just because of the sheer Sp.Atk behind that Aura Sphere, coupled with the fact that Insomnia keeps Darkrai's biggest asset at bay. Having 30 extra base points in Sp.Def helps, as well. Mewtwo Y is guaranteed to OHKO a special sweeper Darkrai, whereas Darkrai is only guaranteed the 2HKO, and none of this is factoring in any Calm Mind boosts. Granted, this is with the particular build that I have. (A Mild Mewtwo Y, I should clarify.)