I reckon 'Echoes: The best of Pink Floyd" would be a good start. It'll allow you to pick and choose between the rather peaky/troughy quality contrast displayed over the years. As mentioned, 'Relics' is a good sampler, but maybe not the greatest starting point, as some of the music is a little dated. 'Dark Side of the Moon', you can't really go wrong with, and it doesn't hold the title of 'Classic Album' for nothing. That's where a lot of people start with Floyd, and you probably won't be disappointed with your purchase. If I may make a suggestion, have a browse of the rather comprehensive discography featured on this very site, but beware of anything released post '79 ('Is there anybody out there' -a live version of 'The Wall' excepted).

Good luck and happy listening ,

Marko

__________________
When Bucks Fizz’s tour bus crashed they all survived. When Metallica's bus crashed Cliff Burton died. Why?