My favorite is definitely "The Rings of Akhaten." Other episodes that I love the music for include (in no particular order):
"Revolution of the Daleks"
"Doomsday"
"Blink"
"The Impossible Astronaut/Day of the Moon"
"The Woman who Fell to Earth"
"The Sound of Drums/Last of the Time Lords"
"The Hungry Earth/Cold Blood"
From Torchwood:
"Countrycide"
"Exit Wounds"
"Captain Jack Harkness"
