Definitely Theocracy, as long as you can tolerate/enjoy Christian lyrics.
If you're truly new to the genre, pick up the essentials:
- Helloween: Keeper of the Seven Keys, Pt. 1 (or 2, they're almost interchangeably good)
- Gamma Ray: Land of the Free (the undisputed best power metal album of all time)
- Edguy: Mandrake (the high point in their career)
- Blind Guardian: Nightfall in Middle Earth (a masterpiece)
- Hammerfall: Glory to the Brave (the album that spearheaded the recent revival)
- DragonForce: Valley of the Damned (whatever you think of their later work--and I actually quite love it--their first album is generally highly respected)
- Rhapsody: Symphony of Enchanted Lands (for the kiiiing... you know the rest)
- Avantasia: The Metal Opera, Pt. 1 (the ultimate power-metal supergroup, an absolute must-have. Pt 2 is also stellar.)
That'd give you a core foundation and the ability to hold your own in conversation with any power-metal fan. There's tons more to recommend--Falconer comes to mind!--but the above are, IMO, the most important albums in the genre.