Totally depends.
Sometimes Emperor with an odd variant or horrible start, Deity when I feel like the usual Ari/Inf slugfest, Sid for a real challenge, Monarch for training SGs and CotMs.
But DG is the most fun level, it still allows for variants, and isn't that tedious for Industrial Age wars. Or, Deity with RaR.
On average that makes DG, and that's what I voted.