Electric/Poison
Grass/Ghost
I much like electric/poison because it defies the unsaid laws that good pokemon can't have a common weakness like stealth rock. (I hate people that assume this must be true) It also provides a unique albiet complementary STAB, and with sketch the possibilities are near endless.
In the way of grass/ghost, I really like it because ghost types paired with an elemental secondary typing (chandelure, jellicent, golurk) are really cool. Grass is a cool unused type as well.
Poison dragon would be cool if dragons didnt screw everything up by being overpowered so often. But very honestly, OU will not be helped by just sticking another dragon in it.