Star Wars
A perfect soundtrack for a perfect movie. I can listen to the score itself and there isn’t one boring or overly long piece of music; in the movie, the music perfectly accompanies the scenes to the point that I can see almost the entire movie in front of me, just by listening to the soundtrack.

Empire
Here the music is much more subtle and intertwined with the visuals, which is a very good thing for a soundtrack, but for me, it lacks the quality to listen to it on its own, some pieces are very long. A reason for this may be that the action sequences in Empire are longer, but not as iconic as in Star Wars. The battle of Hoth simply isn’t a trench run. Just like the whole movie, it may technically be better than its predecessor, but it doesn’t have the same impact on me.
Jedi
For the same reasons as Empire, it just doesn’t have the Imperial March.

Prequels
On the first glance, it’s very similar as with Empire or Jedi, there are a few very memorable tracks and some “fillers”, but while Empire’s and Jedi’s scores work better in the movie than on their own, the PT soundtracks lack the connection to the visuals, they are just background music for scenes that I don’t care much about. For this reason I don’t bother to rate them individually.

I didn’t listen to TFA’s soundtrack yet, so I won’t rate it here.

Lord Haseo said:

equating the music to shit just because the movies are is kind of ridiculous.

I’d say I put the prequels last, not because I can’t disconnect their music from the bad movies, but rather because it’s so easy to seperate movie and music. Musically, the PT’s scores are alright, what tips the scale for me to the OT’s scores is their strong connection to the visuals.
When comparing only the best pieces, I’d say:

Empire
Star Wars
TPM
ROTS
Jedi
AOTC

But for an overall rating, you also have to take into account how the weaker tracks are handled and for the OT, they serve the movies, while for the PT, they are just “there”.