My favorite is Electric/Grass. To be wholly honest, a lot of my preference for this typing does come from the possible abilities, but let's go over what the typing can do stand-alone.
Defensively:
Electric/Grass would be hit 2x super-effective from Bug, Fire, Ice, and Poison, and 4x super-effective by no types. Major STAB threats here would be Heatran, Heracross, Infernape (durr), Mamoswine, Pyroak, X-Scissor toting Scizor, Syclant, Weavile, and arguably Fidgit - I don't play much CaP, so I don't know how often Fidgit carries Sludge Bomb, but I'll play it safe and list him.
The exchange here is that Electric/Grass sports a 1/2 Steel resistance, coupled with 1/2 damage from Grass and Water, and 1/4 damage from Electric. It's no secret that Steel dominates the CaP metagame these days, and since Pyroak apparently sees so little use, something that could catch Bullet Punches with impunity would be beneficial to the game on a whole. Unlike /Bug, there is no Rock weakness, and /Grass offsets Ground weakness, making this typing that much better versus Metagross, Tyranitar, and any of the myriad of EQ carriers.
Offensively:
Electric/Grass is an impressive offensive typing. Hitting super-effective against Flying, Ground, Rock, and Water, its brute strength isn't phenomenal, but it is only resisted by Dragon and Grass. This means that Electric/Grass hits at least neutral versus everything OU with the exception of Breloom, Celebi, and Pyroak. No Dragons resist it, as Dragonite and Salamence are hit neutral by Electric, Flygon is hit neutral by Grass, and Kingdra is hit neutral by both Grass and Electric. In my mind, that's pretty staggering; plus, it solves the dilemma of Grass Knot versus Thunderbolt.
If we do choose an offensive bias for this Pokemon, it'll be pretty hard to stop outright; couple that with certain neglected abilities that Grass can pick up (hey, I ain't sayin'), and it could definitely be a powerful threat.
