My favourite part was the TPK (I know, you're shocked...) But not just because everyone died. It is that every death was meaningful. I thought it was very realistic of the early rebellion, where victories could be had but at great cost. The early rebellion was all about sacrifice for something greater.
And even though I know how it ended, my heart was racing when Vader was ripping through those Alliance troopers trying to get the disk with the plans on it. That trooper realizing his fate, and going from "Help us!" to "Take it! Take it!" was pretty moving.
I also liked how Alliance Intelligence got their hands dirty. Spying is not a "clean and tidy" business. It is necessary, and important. But it is not bloodless. Now, I don't expect Alliance Intelligence to be like a LeCarre novel (spoilers: every "good guy" either dies or fails in some way that is shocking for it's thoroughness). But one of the things that gets sacrificed for the greater good can be moral high ground. I thought the movie dealt with that well.
It also captured well how fractured and tentative the early rebellion was.
