4th Gen Giratina

[Syrex]

I'd think Giratina would be the devil of the pokemon.

He is a different deminsion "Underworld" I'd assume he'd be the devil.

 

[Glaceon_]

Darkrai looks more evil :P

These might be spoilers, so... view at risk of ruining the two spin-off games; Pokemon Ranger: Shadows of Almia and Pokemon Mystery Dungeon: Explorers of Time/Darkness. Also a (VERY) minor spoiler towards the 10th Pokemon Movie, right at the end.

Spoiler:

Darkrai is portrayed to be an evil entity trying to... if I remember correctly, take over the world you're set in, during the second half of the storyline missions. (although most people consider it to be post-game) Its either taking over or just generally destroying it. I'm actually rather disappointed that Giratina didn't make a primary appearance during the story of PMD2.

In Pokemon Ranger: Shadows of Almia, it might not be considered evil, but severely out of control. But during the time before you take it on, it attempts to consume you with its darkness (but fails) and shortly after, sucks some other people into its darkness!

Even in the main series of games, Darkrai is shown to be rather evil, in locking people into eternal dreams/nightmares for; at a guess, fun. The only place Darkrai really gets any slack is in the 10th Pokemon Movie.

Not to mention Darkrai gets a lot of flak (particularly in Pokemon Battle Revolution) for its 'unique attack', Dark Void.