Would this be tier 0?

[gooddragon1]

Wrong. Not only is there a tier 0, but also tiers -1, -2, and Pun-Pun. They are derived from the results of putting builds above tier one against each other in a natural extension of more highly optimized worlds. The fact that you aren't likely to meet more than a moderately competent wizard in-game doesn't mean there aren't things that straight-up find him quaint still buildable with PC rules.

I was always under the impression that tiers above 3 represented flexibility while those up to 3 represented power.

The ability to destroy a planet is insignificant next to the power of the force do 1 million points of bonus damage on any attack is still tier 3. The ability to cast from a small selection of spells (but more than warlock) of your choice is tier 2. The ability to cast from a large selection of spells of your choice is tier 1. I assumed that any spell from anywhere would be tier 0.

The tier thingy doesn't describe power past 3 though as I understand it. A wizard who deliberately prepares and casts no spells ever will likely lose a fistfight to a monk. There may be a tier -1 though which would be pun-pun with create your own abilities dm permission as that is absolutely anything ever.