Blind Guardian are huge, it seems. I'v heard that Helloween are probably the biggest german power metal band, at least in more countries when compared to BG. But it seems to me like BG are the most popular power metal band right now. In the US/Canada, Dragonforce are the most well known PM band and many of these fans don't even know about these other guys. Stratovarius does not seem as popular as they used to be, or at least not as highly regarded in their current state as they once were. And Helloween seems to be coasting by on top, and it's been this way for a long time. It's awesome how Prog/Power metal is so big in Japan. Of course, this is just from my POV. It seems like Guardian have, like, a 'niche' market of the PM community totally in their hands. I live in Vancouver Canada, and if Helloween or BG were to come play here I'd think BG would have a wayyy bigger crowd. But again, IDK. It just seems like Helloween are well known everywhere except North America.
But I really think Gamma Ray takes the spot over Hammerfall. I'm not a fan of Gamma Ray, or even Hammerfall, but I known most consider them the creators of power metal, with Helloween. Even though it was really just Helloween, and Gamma Ray broke off from that. I can't stand Kai's vocals. I'v always felt Weikath was a way better songwriter, and I like his overall style more cuz it's more melodic.
And yes, Stratovarius are definitely an important and influential PM band (in keeping with theme of thread ;p)