In my opinion the easiest song in the game (on Easy) is Stockholm Syndrome from the MUSE DLC. It has 623 notes in total on Easy.
Apart from 2 bits in the middle it can almost be done with your eyes closed. And even the hard bits are probably easier than the easy bits in many other songs.
And the song is worth the 500 MS points even without the achievement possibility.
But any song you are good at on Easy will probably do, I personally found this song the easiest.