[This relates to the PS2 version]
In terms of power metal, you're not going to find it, save for Dragon Force in GH3 and Dio in 80's Edition. For Metal in general, I'd go with GH2, followed VERY closely by 80's Edition. You get Shout at the Devil, Cherry Pie*, Them Bones, War Pigs, Madhouse, Laid To Rest, Beast and the Harlot (if you're into that sort of thing...), Institutionalized (again, if you're into that sort of thing...), Hangar 18 (!!!!!!!!), The Light That Blinds, Six, Thunderhorse, that Push Push (Lady Lightning) song that's strangely addictive, and Trogdor (for sh*ts and giggles).
If you're interested in what the others have to offer:
Guitar Hero: Thunderkiss 65, Iron Man, You Got Another Thing Comin', Symphony Of Destruction, Ace of Spades, Cowboys From Hell, Bark At The Moon, and The Breaking Wheel
80's Edition: Metal Health, Balls To The Wall, 18 And Life, No One Like You, Holy Diver (!!!!!!!!!!), I Wanna Rock, Ain't Nothin' But a Good Time, Round And Round, Wrath Child, Electric Eye, Seventeen, and Caught In a Mosh
Guitar Hero 3: Paranoid, Rock You Like a Hurricane, Black Sunshine, Before I Forget (... if you're into that sort of thing), Raining Blood, The Number of the Beast, One, Closer (don't know if I should include this one), My Curse (if you're into that sort of thing), Take This Life, and The Way it Ends
*Yes, I'm including Hair Metal
I hope this gives you a decent map of the different versions. I'd personally go with GH2 or 80's Edition, then Guitar Hero, then Guitar Hero 3. And I'm giving you a suggested order because face it: You're going to want to buy them all
