It really depends on what you want to make. Here are a few examples:

1. Wind Druid (Hurricane/Tornado) - The most common and very strong.
2. Fire Druid (Armageddon/Volcano) - Great damage, hard to deal with Immunes.
3. Hybrid Druid (Armageddon/Fire Claws) - I hear this one can be good, but I have yet to try it or see one, for that matter.

Give us a clue, and we'll help ya out.