Mijares. I actually never even followed Mijares' division before actually seeing Mijares himself, but it's damn hard picturing someone beating a guy like him in general.
Klitschko @ HW (with his size, athleticism, power, and technicall skill, he could be a threat to anybody) Calzaghe @ SMW (In my opinion, he beats everybody at SMW with the exception of Jones) Mijares @ SFlW (#1 or #2 all time, H2H, at 115)
Klitschko on his best day would beat most ATG's. What is unusual about Klitschko is the difference between his best and his worst performances, were he lost to average fighters. But it is a common mistake to conclude from those losses that a prime Klitschko would be easily beaten by Ali, Louis, Foreman etc. The current Klitschko would be very tough to beat, prime Lennox Lewis would probably have the best chance.
I think there are only a few ATG heavyweights who would beat Wlad, unfortunately. How do you think the Pac Man does?