"Brown Eyes". The song would've been perfect enough with just the instrumentation, but we get the added beauty of Christine's vocals and the sha la las as well! Simply put, there is nothing wrong with that song - nothing about it could be improved.
(I feel like Paul Simon talking about his own song "Graceland", which he thought was perfect. Modest, to say the least!
Although I think he said there was one line he thought might be slightly convoluted, and could've perhaps changed...)