Depends on what kind of power metal you want. If you want old-school, straight up power metal, Helloween is a good way to go, especially their two-part Keeper of the Seven Keys albums. Here's an excerpt, and easily their most well known song.
A good back-up for the classic sound, if you've heard Helloween enough, is Queensryche. They are mostly known for the song "Silent Lucidity", a power ballad, but that is far from their standard sound. They have a concept album, Operation Mindcrime, which I definitely recommend.
Finally, if you want a newer band with a more operatic sound, Nightwish is the way to go. They changed their sound a fair bit after changing lead singers, so I'll post a Tarja Turunen song, as well as an Anette Olzon song. They are on a third singer, Floor Jansen, but I am not familiar with any of their songs with her.
(Tarja Turunen)
(Anette Olzon)