Everyone will have a different opinion, but my vote goes to Destro. Chaos Bolt + Immolate + Conflagrate frontloads a massive amount of damage, and with a little bit of gear that combo will put most enemies below 20% for an immediate Shadowburn. If it does not, it only takes a few incinerates to make up the difference. Most enemies don't even manage to physically reach you.
Ember Tap/Howl of Terror/Healthstone for survivability.
As demonology you get both demonic fury (10% I think) and hp (20% but there's an internal cooldown) when killing a target in demon form... So if you are able to 3-4 shot by overgearing it with heirlooms, and do massive aoe damage if there's a lot of mobs around you, demonology is definitely the best.
However I don't know when you get certain abilities,and even then if you get low uptime in demonology form, it is hard to get full benefits from the spec while leveling.
It's also in general one of the bit harder specs to grasp in general (out of all 33), so if you aren't a quick learner, playing one of the bit more classic type of specs (dotting type of class as in affliction, or big burst damage caster as in destruction) would probably yield a better result.