Best song ever? Impossible to say, there are so many 'best songs', and it depends on my mood, how long I know this song and so on... but there is one wonderful song that still has the same effect on me than when I first heard it, namely Stand By Me by Ben E. King.
That's impossible to say, there are many incredible and bests songs that I've heard in my whole life, but some kinda 'marked' my in different situations, like The Scientist from Coldplay. It's a beautiful one, and I always remember someone special to me when I hear this song.