I just spent a bit of time googling this and a clear answer wasn't readily available, so here it is for future me.

https://huggingface.co/xinsir/controlnet-union-sdxl-1.0

https://openart.ai/workflows/onion/one-for-all-controlnet-union-sdxl/e4tPovxBNxzjtQ19mJW9

In my opinion the best models are canny_pyra, depth, ted, lineart, and normalmap_dsine - but it probably depends on what you're using them for.

Lineart number 1. What are the others even for?

Workflow linked seems great for using a single controlnet, not sure how to effectively stack them - or even if that's beneficial.

(edit: use the promax version)