Pardon my Owl City fanboyism, but.
Any song by Owl City really, but I can't listen to this and not be in awe each time. In fact I can the whole it comes from is one I can listen to over and over again and not be bored.
I agree with AdrianD and say Paradise by Coldplay is another song I can listen to and not be bored, I can find something beautiful about everytime I listen to it.
Along with "Take a Walk", "Sleepyhead", "Cuddle Fuddle" and several others.
Just any song by Paramore really
(Actually any Matt and Kim song never gets old cause they're all too catchy! )
There's tons of other songs, but these are some prime examples.