Ranger goes super fast in the first 60 levels. You can spec into beastmaster, take a cat or something as dps pet. With the signets (toughness +regen for pet) and right traits (like master's bond which increases the pets stats for each mob killed); your dps pet can tank anything but champions.
While he tanks you flank behind the mob with a shortbow and shoot your way to victory.
After that however it totally turns around, because for some reason mobs start to ignore the pet totally.
As for the question though. When the OP says most smooth leveling curve; I'm surprised everyone suddenly dives on what is the fastest.
I can sure as hell grind fast with a thief, but it ain't smooth at all; one mistake and I bite the dust. if I pull an add, i get in trouble, etc etc.
That's why I think smooth leveling classes are the ones that don't just level fast or kill mobs fast 1v1 but are flexible in a lot of situations.
If you compare a necro to an elementalist for example; the latter would flat out win in most 1v1 and AoE time to kill races. However if you get into that hairy situation or if you just need that little extra flexibility; IMO the necro has a bit more to work with.
One of my highest ranking criteria for "fun and smooth leveling" is speed buffs. On my ranger I use my elite (Rampage as One) almost exclusively for the swiftness boon. On my elementalist I absolutely love Ride the Lightning (even though that skill got me killed so often xD). I wouldn't dream of playing thief without Signet of Shadows.
Edited by Nyth, 20 September 2012 - 11:11 PM.