Good NPC Druid builds?

[Applebuck]

So I've recently decided to DM a 3.5 mini-campaign. I've been playing 3.5 for a few years now but this is my first time DMing 3.5, and it revolves heavily around druids, a class I have next to no experience with. Without going to much into the plot, the PC's will have to fight and defeat four evil druids. To avoid repetition, I wanted all four druids to be designed and built differently. For example, one focusing on wild shaping into dangerous animals and another focusing on casting. I wanted to know if anybody had any thoughts or ideas on how to create four druids who would make good boss encounters for PC's and be different enough to make them all different. The PC's are level six and consist of a goliath fighter, a dragonborn/human incarnate, a lizardfolk monk, a human crusader and a halfling rogue.

[Acanous]

There are some druid Prestige classes out there which might fit. Most players don't look at Druid PrC's, because you lose something big in every tradeoff excepting Lion of Talisid (Where you really don't GAIN anything, either) and Planar Shepard.
I'm AFB at the moment, but off the top of my head, check out Verminlord, Blighter and Primevil.

[Diovid]

Using the druid variants from dragon magazine #311 might make interesting evil npcs (especially the Wild Reaper).

A couple of nice feats for evil druids are:

Tainted Druid (http://www.wizards.com/default.asp?x=dnd/re/20030428a)
Child of Winter / Vermin Companion / Vermin Wild Shape (Eberron Campaign Setting)
Beast Totem / Beast Shape / Totem Companion (Eberron Campaign Setting)
Abberation Wild Shape (Lords of Madness)
Corrupted Wild Shape (Libris Mortis)
Feral Animal Companion (Champions of Ruin)
Initiate of Shar (City of Splendors: Waterdeep)
Savage Empathy / Monstrous Animal Companion (dragon #326)