I don't know about the rest of you, but in my opinion, there is only one song to represent Gold. And that would be Ace Of Spades by Motörhead. I could search for the rest of my life for a better theme to sum up Gold. Not only is it all about gambling (one of his hobbies), but it also doubles as a fantastic skateboarding song (another one of his hobbies). Check out the intro for Tony Hawk's Pro Skater 3 and see for yourself. Finally, it mentions other things that Gold is known for ("Playing for the high one, dancing with the devil, going with the flow, it's all a game to me.). Does that remind anyone else other than me of his reckless nature?
For Silver, Lying From You by Linkin Park seems to represent him pretty well, if not perfectly. It's a dark song (reflecting his dark past) that mentions attempts to be left alone. This perfectly represents his early days of being completely independent (other than Blue/Green) and not letting anybody into his already extremely harsh life. It also speaks of performing sinful acts, similarly to his past misdeeds (working against his will for the Mask of Ice, stealing Prof. Elm's Totodile, etc.). As a bonus, it goes well with his initial reaction upon learning that Giovanni is his father.
As for all the other Pokédex holders, I'm still not quite sure as to what their perfect theme song would be. But I'm trying to narrow down my choices. If anyone has any constructive criticism, I would appreciate it if you would let me hear it! I'm always open to others opinions!