Obviously there's a lot of different opinions as to what the easiest song on Guitar is, and those who have access to the internet and downloadable content will have an easier time than others.
As far as songs that are on the disc, We Got The Beat, Pretend We're Dead, Hungry Like the Wolf and Today are some of the easiest songs to get 100% notes hit on.
If any songs are brought over from the Rock Band 1 disc in the disc export feature, Maps, Here It Goes Again and Mississippi Queen are a few of the easiest.
As for DLC, some options are absolutely slam dunks. The most notable is the free song Charlene (I'm Right Behind You), which clocks in at just over a minute long, and only has 146 notes. Several songs from The Pixies' album Doolittle are fairly easy 100%s as well, notable Tame and Monkey Gone to Heaven.