Wow my dad just asked me this very difficult question 2 days ago, so I have recently put a fair amount of thought into this. To me it is all about their first 5 albums, (Roxy Music, For Your Pleasure, Stranded, Country Life, Siren) and in addition Avalon is an amazing album and probably has the best sound quality.
As I told my dad if I was hard pressed and really had to make a decision I guess I would give the nod to Country Life because it is their best album from start to finish IMO. That being said any of the other 4 from the first 5 all have atleast one song that is better than anything on Country Life.
If you are into something more experimental go with either Roxy Music, or For Your Pleasure, this is when Eno was still in the band and his influence is certainly felt throught out these two amazing albums.
Cheers!