What's the best elemental shaman burst rotation?

Well, shammies only really get a heavy burst when they hit 75. Until then, I would suggest Elemental Mastery + LB + Frost/Earth Shock (depending on which one does more damage at your level) as well as thunderstorm at level 70. Essentially use all your instant casts, as until 75, there is no spell that will always crit for high amounts of damage.

As for level 75+, Flame Shock, Elemental Mastery+Lava Burst, Thunderstorm and if you REALLY wanna kill them, Heroism/Bloodlust and spam LB.