Cresselia makes a killer wall.
That's debatable. It's not killer, but it's decent. It really lost a lot of strength after Platinum's debut. Pursuit weakness never was a good thing for her. Cresselia is outclassed by a lot of better dual walls, in my opinion.
Strength-wise, Darkrai completely wins, no doubt. It's placed in the Uber tier, which means it's a top notch Pokemon in the
entire game. It's ability and potential power is unique, but very threatening nonetheless. It gets an 80 Accuracy sleep inducing move - a high advantage over Hypnosis's 60 Accuracy - and the sleep move is coupled with Darkrai's signature abilty Bad Dreams to score damage every turn. It's got this wow Special Attack stat, which it can use very well. It also gains a sort of gimmicky kind of ability with Swords Dance and Pursuit, but Darkrai pulls off the moveset pretty well, nonetheless. So yeah, Special Attack stat
and Swords Dance in it's options can make Darkrai a pretty unpredictable Pokemon, and of course very annoying if used properly.
In a battle together, Darkrai would also win. Not only is it's tier higher than Cresselia's, but it's typing has a huge advantage over Cresselia. Dark type is immune to Cresselia's STABed move, Psychic. Not only that, but Darkrai scores super-effective hits
and STAB in one move. To even add more to that, Darkrai has this move called Pursuit in which it can snag Cresselia and hit it at 120 base power on the switch.
Darkrai also wins the coolness points. It's very mysterious, and after watching the movie it really got me loving Darkrai. I love how Darkrai usually has this negative and evil connotation to it, but in the movie it totally contradicts that connotation and Darkrai actually turns out to be the very opposite of what he
appeared to be. It's intentions in the game were quite evil, though, and very fitting to it's connotation because it does in fact give out nightmares.