Though tornados aren't elemental damage, they are physical.
Somehow they were pumped into the elemental skill tree.
Best elemental would limit you to the fire spells, since hurricane isn't really a damage dealer, but rather a good addon to your tornados.
As for the gear, it's pretty standard.
Helm: either 2dru/3fissure/3geddon or just Ravenlore unique pelt.
Ammy: Maras probably is the best allround, but ofc 3ele gives more raw damage.
Armor: Enigma/coh for +2skills. Enigma gives you teleport, coh gives resists.
Weapon: hoto (or mang songs for more damage and style. no shield if you do though)
Gloves: magefists give fireskills. period.
belt: arach gives +skill.
Boots: whatever tickles your needs. ww's, sandstorms, rares...
this build won't give you nearly as good res/fhr/fcr as the above listed, but since you asked for damage...
Summary: Tornados (wind druids) will be the best and safest damagedealer. They also have easier to cope with immunities than a pure fire druid. But then, fire druids are far more stylish.:azn:
The druid's fire skills are very random, and as a result it's rather difficult to get exact damage estimates. For instance, whatever your LCS says your Volcano is dealing is actually per Ball- I've heard estimates that if you cast Volcano underneath a monster's feet, you'll hit them with anywhere between 20 and 100 balls, which would crank that damage through the roof. Likewise, Armageddon's damage isn't as good as it looks, since it so rarely hits, and Fissure's damage is slightly less than listed per snake (since that's per-second damage, and each snake lasts less than a second), but if you hit with all three snakes you'll be dealing at least twice as much damage as is listed. Oh, and Molten Boulder might deal the listed damage, or if you knock enemies back, it might deal the physical portion two, three, four, five, or more times.
And don't even get me started on Fissure- there's speculation that Fissure operates much like Poison Creeper mats in that it constantly reapplies its initial damage to monsters on a per-frame basis if you get multiple monsters standing on a single vent.
Basically, nobody can really say for sure how much damage you'll be dealing with a Fire Druid. Suffice it to say that, if you load on ridiculous enough gear, you'll have damage numbers to rival a Sorceress.
and Fissure's damage is slightly less than listed per snake (since that's per-second damage, and each snake lasts less than a second), but if you hit with all three snakes you'll be dealing at least twice as much damage as is listed.
With fire skills, you can get armageddon over 15k dmg, and then add infinity to get 30k, fissure can go over 12k with infinity, volcano goes over 6.5k damage with infinity (and hits the enemy 30+ times), thats the best dmy you will be able to really be able to get with an elemental druid.