There is no best leveling spec as a druid. All 4 specs are perfectly viable for leveling.
Resto can quest, but its not as fast as any of the other 3 specs, but its pretty fun if you enjoy healing pvp and dungeons.
Boomy is a lot of fun, and you can multi dot and kill a lot of stuff quicker than feral (cat)
Bear spec has the best survival and can solo a lot of stuff the other 3 can't.
I leveled DS with Resto and Boomkin, geared as resto, and just used boomkin to quest between dungeons. Though you could do it the other way around till level 85. The gear sets are close enough it won't majorly gimp you if you choose boomkin stat priorties and use a resto spec for pvp and dungeon healing.
I leveled as a Cat because then you have unlimited resource and you never have to stop to take a break just throw rejuv on you after every fight and go back into cat form and you can chain pull, you dont even really need to go into stealth as a feral unless you are fighting something strong. Feral is good depending on the BG since they have since target DPS they are great on a FC in WSG or TP.
Affliction is the best, you can take on up to 3 mobs at a time and you own in pvp because affliction spells dont stop once u cast them and even if your dead the spells keep going and can kill a person even if your dead. Druids are good healers and make decent tanks thats about all i know about druids.
For levelling go Feral, you'll experience a lot less down time and stuff dies pretty fast. You can solo quite a lot more than as moonkin (start in cat with cooldowns, dot up, and bear). Also you won't see a lot of balance gear on your way, personal experience when levelling a 2nd druid not long ago.
In RBG feral is also quite nice. Not just for flag carrying (travel form, cat dash, bear), but also to survive quite long when defending an outpost until your friendlies get there (stealth defend, announce when incoming, pounce them just before they take flag and go bear). Quite nice damage against melee classes and lots of armour in bear.
If you look at player statistics there's likely to be more people suggesting Druid as its one of the more heavily played classes. I've leveled and played both classes to 85 with my lock being my main. For druids, I personally didn't like leveling as feral because I like to be able to pull additional mobs while i'm in combat and I found that difficult to do. After around 50 I switched to Balance til 85.
For warlocks Affliction moves pretty quickly. The Soul Siphon and Siphon life talents make you very hard to kill and the more mobs you pull the more heals you get. It's not quite as powerful as it was in Wrath when you could drain tank w/ the felguard at 70 but it's still viable and a little op.
I would personally suggest leveling as a lock. Afflliction or Demo are good ideas especially now that you can get the felguard at 10 when you choose a tree. With demo you can easily nuke down single mobs before they can reach you.
thanks for the replies, i think im going to try druid first since i have never actually leveled one and played one for a span of time. Since i have so much time till release of mop and nothing to do atm i think im just going to level them both.
(Seriously though, I love being a Druid, have been one as my main for 6 years, there flexibility is amazing, and all specs are fun to play (even though I tend to stick to Balance and Resto - thats a playstyle choice though (not fond of melee)
But if you're looking for versatility then a Druid is deffo the way to go!