I played with Akinator back when 4chan was obsessing over him quite some time ago, but don't remember what character I stumped him with then. One that probably wouldn't work now, I'm guessing.
First three characters I tried were Hatsune Miku, Saber, and Arcueid Brunestud, and he got them all right after question twenty. I then figured I'd go with something that a bunch of people hadn't probably already gone with and picked Ulala; he got her after one failed guess (on twenty-four). I then went with Dr. Horrible, who he got after question seventeen. Agent J took him twenty; the King of All Cosmos and Hakurei Reimu each took him only sixteen. I finally fully stumped him with UFO-CO.