electric: as sad as I am to admit it, given that this song is so mainstream that I don't even consider it hardcore anymore, Atreyu's Lip Gloss and Black (*oh the shame

*) ... also, the song has a pretty cool guitar outro... ...
actually, I'm changing my vote... Counterparts And Number Them by Alexisonfire is unbearably hard to play, to the extent that they rarely even play it in concert... actually... most Alexis has super-McKickass guitaring
accoustic: the accoustic part of The Animals' "House Of The Rising Sun"... also, the intro to Drunken Lullabies by Flogging Molly
and I know this wasn't included, but c'mon, there are songs where the bass is super important too! Such songs include (overplayed though it was) The White Stripes' Seven Nation Army, and Feel Good Inc (Gorillaz), and lets not forget, of course, Dazed and Confused (Zeppelin)