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.
I would be fascinated to see the damage breakdown that shows fighters consistently coming out ahead of what both druid and animal companion can do - particularly if they don't exclusively use two-handed weapon power attack.
Druids are no worse at spellcasting than any other core full caster class. They have the same number of spells per day as wizards but know many more. They lose one spell per day to clerics. Their spell selection does not seem to be any more limited than any other class's, except that they never get a
wish/
miracle spell. At lower levels, their healing abilities are not significantly worse than a cleric's.
Druids are great sneaking characters. They can be extremely stealthy by wild shaping into smaller, less obtrusive animals. They can also avoid many traps - almost all in a wilderness environment. They also get 4 sp/level and a solid skill selection.
Druids require only one stat: Wisdom. They can freely 'dump' all three physical stats if starting above 5th level, and as such can have even more skill points or a Charisma high enough to make them decent social characters.
Druids have excellent synergy with each other. For instance, four 9th level druids can wild shape, one of them casts animal growth on the quartet, and each of the other three casts the buff spell of his choice or begins combat. What clerical or arcane buff spell can achieve such results across multiple targets? In short, none - even at higher spell levels.
Druids need few magic items, but benefit immensely from those they get - like Monk's Belts (core, outrageously beneficial), Wisdom-enhancing items, Bracers of Armor, and Natural Weapon-enhancing items.
This is not some "uber" druid build. This is a single core feat (Natural Spell) and the basic class abilities!