Two of their very finest would be "Crime of the century" and the more commercial but still brilliant "Breakfast in America". For more recent(ish) material try "Famous last words", the last album to feature founder member and driving force Roger Hodgson. You could also do worse than check out his solo debut album, "In the eye of the storm".
BTW Supertramp are a 70s band; their first self-titled album was released around 1969 or 1970, so they are definitely not a 60s band. They achieved most of their commercial success and worldwide fame in the 80s, after the release of BIA and the hit singles from that album, "Take the long way home", "The logical song" and the title track.
They were never quite the same though after RH left, although the album "Some things never change" is quite good. Nowhere near up to the standard we'd come to expect up to FLW though...
Happy listening!