If we're talking about their absolute best here, I would have to give props to
Meddle, which is coincidentally also my favorite. It appeals to me on an emotional level and is the threshold album where they began to move towards the "Floyd Sound" they're known for while still retaining a few touches of that raw sort of chaos that characterized them in the 60's. Not a single weak track to these ears either (Seamus included), though Echoes obviously takes the cake.