I think that the OT had way more memorable, signature melodies than the PT (without a doubt), but I still think the PT had awesome music.
As far as memorable melodies, the OT has:
1. Main Title
2. Binary Sunset
3. Throne Room
4. Imperial March
5. Asteroid Chase
6. Yoda's Theme
7. Lando's Palace
8. Luke and Leia
9. Parade of the Ewoks
The PT has:
1. Duel of the Fates
2. Across the Stars
3. Battle of the Heroes
4. Anakin's Betrayal
I still love the PT soundtrack - there are lots of small bits here and there (like the great music in the second half of Anakin's Dark Deeds on the ROTS track) - but as far as really powerful, memorable melodies, I don't think it even remotely compares to the OT.