My favourite song from the Syd Barrett era has to be Astronomy Domine, it's got great guitar and lyrics which are beautifully nonsensical. Also, some of my favourite Pink Floyd lyrics just sound so satisfying to hear sung - think of 'kicking around on a piece of ground in your hometown' on Time, for instance - and this has another fine example with 'floating down the sound resounds around the icy waters underground'. I also really like the extended live version on Ummagumma (although there it is performed by the later quartet including Gilmour).
Gilmour, Wright and Mason seemed to like it as well, as it returned to the setlist for the Division Bell tour.