You've already shown great initiative in embracing all of Metallica's catalog, since that is generally a big poser sneer moment. In keeping with that spirit.
Iron Maiden: Gordon's recommendation is perfect. 7th Son is probably one of my favorite albums of all time, although I would also add A Real Live One as a good roundup of a lot of their best pieces, especially for a casual fan. Otherwise great songs really come to life with psychotic crowd chanting as an additional instrument, with Fear of the Dark being a great example.
I would be remiss if I didn't also suggest Bring Your Daughter To The Slaughter (not a live version). All done appreciating the song itself?
Now there's a meandering NWS live one (4:53 has a great payoff). It took me a while to get past the YayIyayIyaaaay part he tacks on to the beginning in the Real Live One performance as well, that's why I included the studio, and the audio quality is way better on the live album, but rest assured the later improv part intermission is not included in the album. It's just worth noting that some people saw Spinal Tap and said "that's ridiculous" and others...others said "we need to be even more like that, times 11".
Coheed & Cambria: My entry point for their music was their 2nd album, conveniently labeled In Keeping Secrets of Silent Earth:3, and that remains the one I return to even after I've learned to enjoy their other works. Favor House Atlantic is the traditional favorite, but I'd say the IKSoSE:3 itself is my pick (set to FF9 cutscenes, which I guess is as appropriate as anything else if you've seen the Favor video).
It's worth getting physical copies, if only so you can be 100% sure it's not your fault when you can't figure out what the hell is going on in these songs. I'm 100% sure, myself, but you just have to take it as the surreal prog metal it is.
System of A Down: their first album is great. I don't mind their later albums, and it would be a shame to let arbitrary genre snobbery based more on an accident of time and shitty marketing than actual musical quality. That's my 2 cents on that.
Sepultura:Arise. The Arise album and Chaos AD (Refuse/Resist) are great, with the former being a little less immediately accessible but still great. Unless you don't think songs titled "Dead Embryonic Cells" and "Biotech is Godzilla" deserve a listen. The Hunt (off Chaos AD) is probably my favorite song of theirs but it's not what you would call an accurate representation of most of their music.
Slayer: I know, fake playing Raining Blood sucks. I'm not particularly fond of speed metal as a genre, but there is one album I always recommend to people that hate Slayer, and that is Seasons In The Abyss, sort of Slayer's Black Album in terms of hitting a sweet spot between their subgenre conventions and marketability, and it still kept songs like War Ensemble for the purists. Why didn't either of those two make it on to Guitar Hero? I don't know. It kills me, though.
madkevin: thanks for the Baroness recommendation.