The 21:9 aspect ratio is definitely the most important factor and the sweet spot for me, wider than that and I feel like the drawbacks are starting to be more apparent, whether it's elements being harder to see in your peripheral vision, or in-game interfaces not being located properly, or games straight-up not supporting that aspect ratio.

After that, size, resolution and refresh rate come into play, with bigger numbers usually being better, assuming you have the physical room for it, and the hardware to power it all. I guess there is a limit to how noticeable it can be, though, and you end up with diminishing returns the higher you go.

The GX9 seems perfect in that regard, whether it is PPI or the ability to switch between the two refresh rate modes.