I've been thinking about this a fair amount lately. What moments in the films were given more meaning, more emotion, or just generally improved by existence of the films that came after? A big one for me is when Leia senses Luke is in danger in ESB. It's always been a powerful moment, with that beautifully shot close up of Leia and the desperateness of Luke's situation, but the first time I watched the saga front-to-back I almost teared up. Moreso than ROTJ, this is the moment where they realize they're siblings, even if it isn't conscious knowledge yet. Seeing that long-awaited connection happen at such a crucial point and in such a poignant manner is fantastic. Not to mention, I think it forms a great parallel with the "ruminations" scene in ROTS.