I would recommend cows since there are so many of them in a relatively small space.
so assuming gem drops are about the same for all monsters you'd get the best gem drop density based on monsters/square foot ratio in cows.
heck, just norm cows to make it faster. just don't kill the king so you can keep gem-farming. might be a good idea for low-rune farming on ladder too. I think I'll start doing that soon.