Elemental is a range Caster spec with high burst and high survivability making this the most ideal leveling spec for Shamans. This spec will allow you to solo quest very efficiently and also play as a healer for Dungeon runs because you will collect caster gear but you will need to be a bit more mana conservative as this is not a healing spec. Elemental spec also offer he best AOE talents for multiple NPC grinding.
Enhancement spec is an awesome solo leveling spec with high mobility thanks to Ancestral Swiftness offering High Single target DPS but you won’t be able to play as a healer and will suffer longer queue times for Dungeons. AOE grinding is riskier than Elemental Shaman as you take considerably more damage because you loose your Shield for Dual Wielding.
Restoration spec is a very poor spec for solo questing, but great for end game Raiding and Dungeons.