Angra (sheer Brazilian genius, especially post-Matos, which goes from the Rebirth album and on)
Gamma Ray (the third band in the power metal genre, and one of the original 4 pioneers - was started by Kai Hansen, former guitarist & vocalist of Helloween)
http://www.youtube.com/watch?v=Dxe-_o5FMIU
Helloween (by far the biggest pioneer of power metal, as well as being the second band to enter the genre)
Blind Guardian (another one of the original 4 pioneers of power metal, mixed a blend of speed metal with power metal, although more recently they've gone more progressive)
Manowar (heavy metal, one of the giants of metal that tends to get overlooked)
http://www.youtube.com/watch?v=J1b_9A2EFyA
Symphony X
http://www.youtube.com/watch?v=3RUO-vHlKc4
Kiko Loureiro (Guitarist for Angra, has some amazing solo stuff - his guitarwork is influenced by his love of Brazilian jazz)
Nightwish, as much as I love them, isn't a band known for having the most amazing guitarwork or speed - its a band that gets attention for how it melds a female lead vocalist (especially formerly when Tarja was the vocalist) with great arrangements in metal.