I voted Chaos. Not surprised that the other 3 votes were for armies that don't generally have much, if anything, in the way of decent proxies available.
Most human/elf/orc/undead units are readily available already. Although there are gaps, Dogs of War Birdmen for example.
I'm surprised Chaos Dwarf isn't on the list but Eureka cover that pretty well, apart from Hobgoblin infantry types and maybe bolt throwers and their crews. I might, one day, build a Chaos Dwarf army, if only because I have loads of orcs/goblins already.
If I was a manufacturer looking to fill gaps, I'd take a close look at what is already available and initially do one army with gap-fillers for other armies if I could be assured of sales.
Edit: Having said that, Pendraken do seem to have a lot of stuff on their website, it's just the lack of photos that I find a bit off-putting. I want to see what I'm buying before I part with my hard-earned cash! If Pendraken could address that issue, perhaps we'd find that there are more proxies available than we think!