My favorite songs are- Tenshi ni Fureta Yo! Fude pen, ball pen Gohan wa Okazu U&I No Thank You! Fuwa Fuwa Time Utauyo! Miracle
I think the songs Mio sings were often better, like Don't Say Lazy, No Thank You! and Singing! They are definitely my favourites since the guitar parts are more fun to play (I'm trying to learn to play all the K-ON! songs on guitar), but Tenshi ni Fureta Yo deserves a special mention ever since I unwittingly cried when I first heard that song in the anime. Also Ritsu has a surprisingly lovely singing voice in that song and aside from her character songs it's the only time she ever sang.
My favorite songs are the movie ending and Tsubasa wo kudasai.