Seen as how acoustic music is just about my favouritest sort in the world, let's just restrict this to one and absolute acoustic classics by my some of my favourite groups.
"Norwegian wood"- The Beatles
"Lucky Man"- Emerson, Lake and Palmer (ignore the Moog solo)
"Horizons"- Genesis
"Friend of the Devil"- Grateful Dead
"Mother Goose"- Jethro Tull
"Going to California"- Led Zeppelin (Tangerine, That's the way, Gallows Pole, Bron-Yr-aur and Bron-Yr-Aur stomp are all crazy good acoustic Led Zep songs)
"Old Man"- Neil Young
"Wish you were here"- Pink Floyd
"Love ain't for Keeping"- The Who
"Mood for a Day"- Yes
That's not a bad list.