I only discovered Pink Floyd in the mid 80's, so I didn't hear a lot of their early stuff. I had heard the single "Another Brick in the Wall" sometime about 1978 [??], but nothing else.
The first LP (yes, it was still plastic 33s in those days) I heard was Animals, which I consider to be my favourite, in 1984. In the next year or so, I listened to everything they'd recorded, up to and including "A Momentary Lapse of Reason" and Roger Waters' "Radio Kaos" (yeah, I know he'd left by then...) and I even went to Wembley Stadium with a mate in August '85 to see that concert (still got the tickets in my scrapbook).
However, that was only about 20% of my education. Under the influences of certain substances, I started to try to work out what they were singing about, and that's when I realised that Pink Floyd songs are not just a nice melody you can click your fingers and dance to.
Animals is my favourite because I can identify with the kind of people that are being described. Dark Side Of The Moon is a bit weird, because I'm not a nutter, and Wish You Were Here... Well, I'm happy where I am now, thank you very much...
EDIT: If you ask me my favourite single song/tune, the answer would be "Time".