I guess I hadn't voted on this before, so I picked Astronomy Domine, since that was the second Floyd I ever heard, and it totally got me hooked on them. If you want to hear psychedelic, just listen to that song, and you're taken to another world. I think Interstellar Overdrive is a little overrated, I can't get into it much past the initial riff. I thought it was an interesting little piece of improvisation, but then I heard the Velvet Underground's "Sister Ray," and decided that Sister Ray totally wipes out IO in terms of noisy freakout-ness.