In my experience and in most of the theoretical work I've seen, the druid is clearly the most powerful overall base class in 3.5 across most levels, with the possible exception of Eberron's artificer. Ironic, that the most and least natural classes represent the pinnacles of power.
Anyway, here's where I'm coming from on this.
Beyond 5th level, Druids are, at worst, as good at fighting as fighters. They may lose a few points of average damage per round at times, but increase their damage output tremendously for every additional round of preparation they have. Since they can summon tons of creatures to distract or delay enemies, they typically get more time to buff themselves than other casters would.
For example, a 9th level druid can become, in two rounds, a Huge dire lion with a Strength bonus of +11, the ability to almost automatically win any grapple check against a CR-equivalent PC-race character (and most CR-equivalent monsters), and an attack routine of 2 claws +15 melee (1d8+11), 1 bite +10 melee (2d6+5). That second round of buffing also gives the druid's animal companion a huge power boost - perhaps making it into a Gargantuan giant crocodile, for instance. In one level, the druid can have a dire lion and buff it the same way he buffs himself, if he so chooses.