I am working with a render, which shouldn't be that hard on the denoiser, nevertheless, well, I think this image says enough:
This render was after a hundred samples, and the noisy render it is based on doesn't look that crazy noisy to me:
So far I think I have tried every denoiser option there is. Openimagedenoise, optix, albedo, albedo and normal, none, cpu render, gpu render (cuda), accurate, fast. I don't think I have tried every possible combination of those options, but I have tried a lot of them, and the results are always very similar. Also, my normals look fine.
Edit: add blend file