I can't really say for sure what my perfect song is. I like too many genres that just usually don't gel with one another and that I all like for different reasons.
But from each genre, I have a perfect song that is an example of what I like out of that genre.
Rap/Hip Hop
This song has such monstrous energy behind it, a frantic pace, and lyrics that don't just flow well, but actually make sense in a poetic way.
Punk
There's way too much variation in the punk I like, so for brevity sake, I'll just put one.
Good pace, light but meaningful and clever lyrics, and uncharacteristically well played(take into account the lead singer is also lead and only guitar, and he's capable of a lot better).
Folk
When you absolutely must depress yourself, accept no substitutes for Irish folk music. Far Away Boys by Flogging Molly, Foggy Dew, Green Fields of France(not necessarily Irish, but has been given that flavor a few times).
A nice mix of modern and classic instrumentation carries a just touch different flavor to a good old song, though it's depressing as fuck.