Oasis' Unplugged & Unliammed is my favorite. I belive it's only available bootlegged... For studio albums, it'd be something by Bob Dylan. John Lennon's Acoustic is awesome too.
any bob dylan all acoustic album from the mid sixties plus "blood on the tracks" and new morning by dylan.
you guys should check out a musician called Don Ross, he won the u.s freestyle finger picking championship twice for creativity, he's the only person to havde done that. his solo acoustic works sound like ten guys playing at once...seeing him live is mind boggling. he blows the shit outta any artist i've ever heard before.
i love nirvana unplugged but my all time favourite acoistic artist (and hes won a shitload of awards for it) is john butler.... damn he is good!! he was born in california but moved to pinjarra when he was 7 (pinjarra is in northern territory here in aus) and he got his grandads dobro when he was 16... theres too much to say bout him but i strongly recomend going onto www.johnbutlertrio.com or going out and getting one of his albums cos they prolly will blow ur mind (hes got bout 5)