The Itza Empire. Because they were the biggest Maya state that could actually be called an empire, and because I thought it'd be funny to see the reactions of people used to dealing with corrupt cops and authorities now having to answer to some weird dude calling himself "Lord Chac" and his council of ministers.
I second the lovely and tragic Austria - Hungary
The Habsburgs were just, devoted to their subjects, and they just plain
had an awesome sense of style! Coolest uniforms ever!