Garchomp.
And an Dragon/Rock type still doesnt exist.... actually, a Rock type legendary doesn't even exist at all.
Same with Fighting, Bug and Poison.
Yeah... there's no point in ignoring the other types. Darkrai was like the first pure dark legendary, and dark type for that matter. I think it took them WAY too long to get a dark type to become legendary.
P.S. Darkrai is Chuck Norris in pokemon form. Period.
And don't forget ghost, Giratina was the first Ghost type legendary.
I like Dark and Ghost types, not because they look ''cool'', but because they're affective against Phychic and Ghost types.
Dark's only weaknesses are Bug and Fighting, but there are not much Bug and Fighting trainers in any pokemon game yet.
Ghost is a cool type because Ghost pwns other Ghosts, and it murders Psychic pokemon too.... the only bad thing is that it does the same in reverse.
Psychic is cool too, i have an Alakazam with Light screen, Calm wind, Recover and Psychic, that's the best moves for him imo.
The bad thing about Psychic is that it's hyped, and Psychic isn't really affective against other Psychic pokemon.