Best score, like all music (and all art, for that matter), is incredibly subjective.
When I saw DEH, I was so caught up in the story and in Platt's performance that the music sort of passed me by. I knew that there were songs in it that I liked, but the music was not the highlight of the show for me. However, because I thought the show was so good I downloaded the cast recording, and the more I listened the more addicted I became to the music, and now it's one of my favorite scores.
I thought that the music in Comet was inconsistent. Some of it was truly exciting, e.g. the Russian dances, and many of the arias were beautiful. But, the rest of the music was, to me at least, dull. I understand that it's recitative, but because of that it's not a score I'd listen to over and over again.
So for me, it's DEH. Many will disagree and that's fine.
CZJ at opening night party for A Little Night Music, Dec 13, 2009.