I went for No Way Out, Growing Up and Sky Blue.
My first impulse was Signal To Noise, but I think the big drama of this song is starting not to age well.
My second impulse was I Grieve. While it certainly is an outstanding song that depicts the numbness that takes hold when you lose someone very dear as well as the final reassertion of life, I cannot in good faith call it a "favourite" track on UP.
Had the question been for the most accomplished song on UP, the ones that do what they do better than the other songs do what they do, I Grieve would have been first by a long margin on my list.
No Way Out is, perhaps, my proxy for I Grieve. I like Growing Up for its general "life is a funny old thing" stance and the upbeat music, and Sky Blue for its meditativeness (if that isn't a word it should be).