I don't know, it changes for me but I do think that High Hopes from their last studio album (Division Bell) may be underrated. It's a lovely melancholic song with a fantastic solo at the end by Gilmour and it feels a bit like a goodbye ..
I get the feeling so many people disregard the post-Barrett-Waters Pink Floyd and those should really check that one out.
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[youtube]Bqvcmud3LFQ[/youtube]