Like many, my choices change quite often...not only day-to-day, but sometimes minute to minute.
If I were pressed, right now, I would say Sonic Youth's Washing Machine. There is something about the song...going from Kim Gordon's very ugly vocals at the beginning, moving to an almost Doors-like performance in the middle, then finishing with some of Thurston and Lee's classic guitar wrestling.
It took me literally days for me to convince a friend of mine that the lyrics "Someone left the cake out in the rain, I don't think that I can take it, because it took so long to make it, because I'll never have that recipe again" exists. I had to find an album at a used record store and play it for him before he was a believer.
What is worse is that I am able to remember those lyrics without looking them up.