...and by that I mean the song playing when you see the title and the "Press Start" text.
For me,
http://www.youtube.com/watch?v=_0BD_bh6b2Q
Some of my favorites have already been mentioned, so here's this one:
Prince of Persia 2008
Yep, another vote for Xenoblade here.
The FTL theme was also pretty nice. I'm going to put out a mention to Trine 2 though: http://www.youtube.com/watch?v=Znw_wi39nsQ
Also the original Trine: http://www.youtube.com/watch?v=ff-VeoumJQU
Double Dragon Neon- http://www.youtube.com/watch?v=jxgwZiCM6wA