Well all of them are made of awesome, but come on! there has to be one that just beats the rest for you.
For me it was Accio Deathly Hallows,
if it not for it being featured on YouTube i don't think i would be here right now.
"I'm Gonna Kill You"
"Dum de Dum de Dum de"
"It's Too Hot"
Or course I love "Accio Deathly Hallows" and "The Man Who Throws the Tetris Piece" (mainly because of the party blower solo) but they were the most publicized...and that just kinda turns me off. The songs I mentioned first are kinda obscure compared to the the two latter but they just get stuck in my head so easily.