Mine (The Humbling River by Pucifer) was selected in no small part because of the haunting melody. While none of you know Longshot yet, I'm certain you will find he's rather firey and opinionated, which made Linkin' Park's "A Light That Never Comes" an early front runner in my considerations. But that side will be readily appearant once you start interacting with him. So I went with a tune that better reflects his mental processes.
The lyrics are no small part of it however. I can go line by line and explain how each part applies to Longshot personally, but that's a lot of effort just to get lots of tl:dr responses. (I'm risking that as it is.) So let me draw your attention to lines two four in this particular stanza:
Angel, angel what have I done?
I've faced the quakes, the wind, the fire
I've conquered country, crown and throne
Yet I'm helpless by the river
Line two is about three of the four classical elements (earth, air, and fire) with the fourth (water) being represented in the last line. To the ancients however, these represented more then just the world around them; they were what made up a human being. Earth was one's physical body, air was their intellect, fire their will, and water their emotions and subconscious. The first three are conquered by the singer while the last remains an insurmountable obstacle.
This is Longshot in a snap shot. His body is a weakness of his, in truth. Complications at birth and a run-in with a heavy dose of 'radiation' (actually residual necromantic energy in his world) left him particularly weak and frail. He learned to bend it to his will where he could and compensate with his magic where he couldn't. His intellect and will are both sharp, and further honed and focused by training, disipline, and experience.
Water though, his emotions and subconscious, remain the one thing he cannot get past. And it's because when in combat he can simply shut them down, much like how he shuts out pain, and so they don't unduly affect him in the one pursuit he's honed himself for. Outside of combat though, he doesn't have a good grip on them. He likes to think he is ruled by his principles and reason, but his emotions often take the reigns when he's pushed outside of life threatening situations.
(The stanza that preceeds this one is in a similar vein but uses the eastern elements (substituting stone for metal) which I'm less familiar with, so I went with this one.

I hope you enjoyed this little peak into Longshot's (and my) mind. Tell me what you think and tell me about yours!