From my personal experience of soloing all three, I would have to sayu that they are prety much equal. The totemic druid may be good early on, but towards the end he/she starts lacking. The Shapeshifter can be a very good fighter, but you can' cast spells... I guess that isn't really different than a regular fighter. But, for a SoA *and* ToB game, Avenger Druids are the best.
You can easily use a bunch of the spells to create devasating sombos. such as casting web, then shapeshifting to a sword spider to kill your enemies while they are imobile.
The penalties don't really matter, especially the CON minus. Druids only need a 16 Constitution to get the max hp bonus, anything above that is useless. and a 16 Strength is pretty good for a druid anyway.
So, for full SoA, ToB soloing, Avenger. Just SoA, Totemic. Shapeshifter for some spice
