When I think of a perfect prog ballad, full of true emotions and a wonderful melody, one of my first choices surely is "Up to Me", the closing track of the first side of Jethro Tull's "Aqualung". Even if the musical theme has a mellow and somehow folk flavour, there's an underlying tension all through the song, provided by Ian Anderson's powerful vocals and Martin Barre's guitar.
Mostly acoustic as it is, "Up to Me" is a powerful, biting song, also graced by a beutiful flute riff. If you can put so many things in a three minute song... well, you're great. And of course that's what Jethro Tull are.