OCI and Systematic Riffage are probably the best people to ask about power metal.
Like someone said you can't have Power Metal without clean vocals, well you can but it's rare I suppose)
I don't consider myself super informative on Power Metal, but the 7 you must listen too IMO are:
Blind Guardian
Rhapsody (Of Fire)
Angra
Edguy
Hammerfall
Helloween (Mostly the 1st 2 albums, but the 2 latest ones are also pretty good)
Sonata Arctica
As far as the rest go here is my list:
Adagio (I am hooked on this band, this band is pretty along the lines of what you're looking for)
Amoral (The "Show Your Colors" album)
Angel Dust (More Speed Metal, but there later albums are Power and it's quite good)
Anubis Gate (Similar to Mercenary)
Ari Koivunen (Fuck all the haters, I actually think he is quite good)
Assailant (Specifically the "Wicked Dreams" album)
Avantasia (Tobias Sammet's other project)
Bob Katsionis (Ex-Firewind's Instrumental project)
Dark Moor
Delain
Demons & Wizards
Dignity
Dragonland (I'd highly recommend checking this one out)
Dream Evil
Elvenking (mostly the earlier albums and Demo's)
Evergrey (You could consider it Power Metal, but more on the Progressive lines)
Fairyland
Falconer
Firewind (Great Power metal with Gus G on the Axe)
Freedom Call
Gamma Ray (Kai Hansen former vocalisr of Helloween plays in this band, check these guys out for sure)
Grave Digger
Hibria (They are full of awesome
)
Imperanon (More on the lines of CoB)
Iron Fire
Iron Savior
Kamelot (Like other people have said (Epica) is all you need)
Kotipelto (Stratovarius' vocalists solo project)
Lost Horizon (Krigloch will agree)
Luca Turilli
Magica (Female Fronted)
Manigance (Pretty good Power Metal from France, even though the lyrics are french)
Masterplan (I'd highly recommend checking these guys out)
Metalium (One of my favorites)
Mystic Prophecy
Nightwish
Nocturnal Rites (To me they have one of the best Power Metal vocalists)
Pagan's Mind (Please check these guys out, they covered a King Diamond song probably the best I've ever heard)
Pyramaze
Serenity
Sinergy
The Storyteller (You'll surely like this one)
Stratovarius (albums from 1998 to 2003 are there best IMO)
Symphony X
Thy Majestie
Warmen (CoB's Keyboardist's project, good stuff)
I think that's pretty much it for me that I thoroughly enjoy, it's upto you to decide what you like in that list, I'm sure some people can suggest some more, but this is my two scents.