What is the name of song that a remix all best songs from 2009?

The name of the song you are referring to is "I Gotta Feeling" by the Black Eyed Peas. It was released in 2009 and was a global hit, topping the charts in many countries. The song is a mash-up of different popular songs from the previous year, 2008, and includes references to songs like "Low" by Flo Rida, "Right Round" by Flo Rida, and "Boom Boom Pow" by the Black Eyed Peas.