Brandy & Monica - 'The Boy Is Mine'
I have to include this although I have mentioned it in a few 'best of' lists in the past - but I do play it and play it again. That first 'harp' phrase - trying to catch how it is done, how it was put together. And then the countermelody of the strings, I just never get bored with it. The interplay between the two singers - the phrasing that each manages so masterfully - also has me fixated.