I thought TPM had several memorable themes and underscoring; if I could listen to it without associating it with the movie, I might like it more. AotC had a decent love theme, but I don't own the album. There's a good-enough statement of that theme in the next movie. RotS had lots of good action music and some genuinely creepy stuff -- I'm thinking about the opera, the Night Skies-like scene, and the dirge segueing into the Imperial March (shamefully absent from the CD!).
This is going to sound kind of contradictory, but here's some of the things I didn't really like about the prequel scores:
1. Reuse of themes. I wanted to hear a new main title theme, to differentiate the PT from the OT and to maintain the integrity of the Main Titles as Luke's Theme. Imagine, ten years from now, some kid watches the PT and kind of likes it. Then he puts in Episode IV, hears a completely new, bombastic, exciting score and reads the words, "A New Hope." That could've been awesome.
Some of the music reuse seemed inappropriate. Duel of the Fates playing during Anakin's swoop ride, or the ESB stuff tracked into the Yoda/Palpatine fight.
2. No reuse of themes. Where was the Boba Fett motif? The droid motif? The original Imperial Theme? This was the perfect opportunity to develop those. At least we got to hear neat things happen to Jabba's and Palpatine's themes. How odd that the most stirring march we got out of the whole thing was the Flag Parade!