Grass / Ghost
Poison / Dragon
Electric / Poison
Electric/Poison= 4x Ground weakness. Bad idea IMHO. I don't see any use for it.
Psychic is better than Elec/Poison, but boring. Only reason is to steal Mewtwo's signature move +STAB.
Poison/Dragon will bring some beautiful artwork, but poison is a crappy type to have. I can see this be cool, but Dragon is not very interesting to make a CaP about.
Grass/Ghost is my prefered typing for 3 reasons:
-Ghost means spinblocking. It gives imunnity to fighting, including HiJump Kick. That's always interesting.
-Grass is a great typing in OU, because it takes on a lot of pokémon I'd like to see taken down. Politoed, Tyranitar, Swampert, Gastrodon, Jellicent, Starmie and Terrakion are all weak to Grass. Not to mention the amount of pokémon that can't really touch a grass type. I disagree with Rising Dusk's linked post, as there is quite a lot of common pokemon that check grass types. Pokemon like Ferrothorn, Forretress, Scizor, Celebi, Heatran and Skarmory all have no issues with Grass attacks and are very common pokémon. That doesn't mean that Grass isn't an interesting typing. I can't be the only one seeing STAB Frenzy Plant as a LOLmove.
-Awesome artworks. Awesome artworks. Awesome artworks. Gamefreak dropped the ball on Grass/Dark pokémon so far, I want a scary plant!