Stairway To Heaven by Led Zeppelin

Just obscure enough to make what you want of the lyrics; just awesome enough to want to learn every part on the guitar; just long enough to be too long for radio.

Bohemian Rhapsody by Queen

All the same reasons as Stairway, plus better singing. This is one of those songs that never ceases to give me chills every time I hear it.

Say It Ain't So by Weezer

Perfect belt-a-long song. It's ready-made for karaoke, and plays well on Rock Band. I recently participated in a one guitar, 12 person sing-a-long of this at a party, and it was a uniquely human experience, to say the least.