What's the most powerful 3.5 core build?

[ericgrau]

You can buy a riding dog too but if you can buy one you can buy a heavy warhorse and the heavy warhorse is much better.

Yes, eventually both riding dog and warhorse are options to consider. But a riding dog isn't affordable at level 1.

The Guard command is not restricted by attack's type restrictions. Guard attacks anything that threatens you or the animal.

Yeah but guard has other restrictions:

Guard (DC 20): The animal stays in place and prevents others from approaching.

Trying to use guard as a substitute for attack sounds fishy at best. Moving behind your animal and saying guard isn't that bad of an idea against melee foes. But many foes have ranged attacks, including many primarily melee foes. Even for melee charges attacking sooner is better than attacking after waiting for the enemy to approach first. Especially at low level when that first attack might drop the companion or force him to flee. Teaching him to attack all creature types is ideal, and having the ability to attack the 4 common creature types before they approach is much nicer than waiting. I can see why a druid might make his first trick guard instead of attack, but it's still far from ideal. And guard isn't necessarily the better choice out of the two.

Also arguably an animal companion or even a friendly dog might protect its owner even without any tricks, though only after he was attacked or otherwise realized his owner was threatened. He most likely will protect himself regardless.

With modifier 3 I would assume that a gestalt druid/wizard would win, given the "druid vs. wizard" battle in this thread.

Sounds great at high level, but that's the easy part. Besides the need for rez help from even a single failed save at high level. If this is meant to survive low level like 1-4 or perhaps even 1-7ish, maybe gestalt wizard with something more stabby like barbarian & fighter. I mean druid is already a little bit of a gish in a can, so gestalting with a full caster is partially redundant. Sure it gets you more dakka later on, but that may not be the issue. And all this assumes "all SRD" includes optional rules like gestalt. It may not.