OK well here's my ratings (based on the OP's rules).

TPM - 5

AOTC - 3

ROTS - 6

ANH - 6

TESB - 8

ROTJ - 7

I've watched all six films in a row (not on the same day of course, lol) and I have to say that none of them were "bad" so to speak, as I enjoyed all of them as part of the saga. AOTC was the worst IMO because at some parts I wasn't really into it, and it seemed a bit dull to me. The entire saga would still make sense if AOTC was just taken out, it didn't really add anything to the story (I guess the origin of the stormtroopers, but meh).

I gave ROTJ such a high rating because even though the majority of the film was mediocre, It contained my favorite scene in the whole saga, which is the final lightsaber duel between Luke and Darth Vader. I also like Palpatine in ROTJ. That lightsaber fight was the best of all of them (even though the duel in TPM was a close second). The score was brilliant, and the fight was just well executed.