Personally, I think Vow of Poverty works best for a druid. All that AC bonus while you're wildshapped? All those stat boosts? All those Ex abilities you gain could be debatable on whether you keep them while wildshaped, but since they're level based gains, they should be fine. So if you put the +8 into Str, +6 into Dex, you can have a Dire Bear who is pretty hard to hit. THen you animal growth yourself, and you're at +16 Str on your dire bear form. It'd say that's pretty decent.
Druids can also fly rather easily, and a whole bunch of other things. Imagine casting entangle, then wading in without fear of getting yourself trapped. And eventually having Elemental forms. Then there's how Barkskin works now.
A Monk/Cleric with the Plant domain has possibilities as well.
I'm about to start a Monk, that will more than likely go into cleric. I think it will work well, since I can cast spells like Spell Resistance, and others on myself. Any energy resistance, and the ability to heal myself easily.
Daag