SteelBagel

Title. Would it possible for say, a horse to bond an Honorspren and become a windrunner, assuming the spren isn't being upity about bonding an animal? How intelligent would an animal have to be? Could a plant do it?

Allomancy is out because without compounding, it has a cap. there's only so hard you can push, so much pewter you can flare. Vin or Elend are about the max you can get using Allomancy alone and nothing else, and they are weaker than someone like susebron or ishar. Feruchemy stronger, but it has limits on duration. When you get to the point where you start to equal stuff like an elantrian fireball or a dustbringer Division surge, or stormlight Healing, you're going to run out of juice and lose the endurance battle without compounding. And most surgebinding is just not versatile enough. This is assuming you only get 2 and haven't bonded like 5 different spren. 2 surges can't do as much, and most surges aren't as good as other methods of doing their same deal elsewhere. For example, transportation & gravitation don't give as much mobility as elantrian teleportation. Cohesion and Tension don't shape as well as forgery. Nothing we know of in surgebinding has the range and utility of Awakening afaik. It just gets beat out by the top levels of other stuff, although I think most surgebinders are on average some of the most powerful magic users in the cosmere.

It's probably theoretically possible they could, especially now that they're unchained, but lightweaving requires Intent and Connection, and they don't have any of that to radiation because they can't sense it or think about it to Connect to it, and they don't know about it to Intend to make it. Also, gamma radiation and Ionizing radiation has way more energy than visible light, which means it's going to take more investiture to make into energy, which means it's going to take a lot of stormlight. Possibly so much that it's virtually impossible. However, the ROW Ars Arcanum says that lightweaving is "The Surge of Light, Sound and Various Waveforms", so it is at least theoretically possible afawk.

Well If we're going with single magic system with no crossover magics, allomancy, feruchemy and most surgebinding is out. We've never seen the upper limit of Awakening, since we only see a few minutes of the god king, and he's probably not very good at awakening, since that was mere moments after he got his powers. And theoretically you could get even more breaths than that to get even more hightenings. Hemalurgy is powerful, but it's extremely hard to use, and nobody is really good at it. Also, overuse opens you up to mental effects. Plus it seems pretty limited as for what you can do without copying other magic systems or going overboard in spikes. Bondsmithing is also pretty powerful, but I just don't think we understand it well enough to justifying it to rank it #1. Aons are super versatile and powerful, but region-locked. It really depends on where the battle is taking place and who it is fighting, and how well they understand their powers.