While the CMC theme song is obviously terribly sung, with lyrics that don't make sense, the instrumentals and overall melody absolutely SLAP
Nobody talks about the Heart Carol, since the version from the show is so short, but if you listen to the full version on the Christmas album, it's absolutely beautiful!
On the topic of Hearth's Warming songs, The Seeds of the Past gets overshadowed a lot by the other great songs in that episode, but I love it because of its sentimental tone and the fact that it conveys a lot of emotion in a short period of time.
I like Glass of Water a lot, not very tuneful, but there's a lot of fun lyrics and gags packed in there.
Rules of Rarity seems to be overshadowed by Art of the Dress, but I actually like it more. I like the constant change in emotion and tone, and Rarity's voice in that song is beautiful, especially the ending of the first part of the song.
I've heard The Place Where We Belong get dismissed as another generic friendship song, but I love the harmonies and instrumentation, and it has very sentimental lyrics.