I'm passing it to the old-as-existence grandmaster of martial arts with autonomous reactions, immortality, self-sustenance type 3, gravity manipulation, existence erasure, power nullification, non-physical interaction, soul manipulation, matter manipulation, time manipulation, sealing, probability manipulation, statistics reduction and...well, I can keep going but this is taking too long.
Point being, Darkrai is nowhere near the skill-level of the Grand Priest and most of his abilities can be negated or resisted whereas Darkrai can't resist most of the powers that GP has.
To add further, would Darkrai even be capable of surviving a single hit by the Grand Priest? Its movie version died to two separate attacks by Dialga and Palkia (albeit it managed to survive for a period of time and stop the attacks from destroying spacetime) whereas the Grand Priest should be the 2nd strongest 2-B in all of Heroes, when the weakest 2-Bs of Heroes are already nearly infinitely multiversal.
Based on my perception, it seems like Darkrai would nearly die from one hit by GP, and if this is the same Darkrai from the films he only has spherical Dark Voids, meaning GP will just auto-dodge everything with ease. If we are counting the capabilities of PMD Darkrai, he might have a shot at using near-undetectable sleep hax, but GP has endless ranged abilities to take him out first, and can likely resist sleep manipulation.
Overall, voting Grand Priest.
