In terms of just raw panel resolution, you're probably looking at the Pimax 8K, HP Reverb, and Index.
The Pimax spreads its pixels over a much wider field of view and upscales a 1440p image to each of its 4K panels. The Reverb has a narrower fov but higher pixel density. And the Index is stuck somewhere in-between.
Beware that all have their quirks and issues before you buy (do some research).