I suppose "easiest" will vary a bit, depending on which techniques you are able to master.
As for me, I never touched a guitar before picking up RS2014 in early January. These are the songs that I have made the most progress in:
97.7% "Next Girl" - The Black Keys (Lead). A lot of repeating phrases. Whole song is played on 3 strings, at 3 frets.
94.9% "Blitzkreig Bop" - The Ramones (Lead). 3 strings, 4 chords. Moves quick, though. Had to move away from this until my strumming/fretting improved.
94.5% "Go With the Flow" - Queens of the Stone Age (Rhythm). 3 strings, 4 chords once again. Also very quick, but with very little variation.
87.9% "Six AM Salvation" - Versus Them (Rhythm). Not all that easy, but I really enjoy the song, and have put a lot of work into it.
82% "Angela" - Jarvis Cocker (Lead). A lot of repetition on single notes. I could probably get this one in the high 90s if I worked at it for an hour.
81.7% "(I Can't Get No) Satisfaction" - Rollong Stones (Lead). This one just seemed to click for me.
81.7% "A More Perfect Union" - Titus Andronicus (Rhythm). Another not too easy song that I've put a lot of work into.
As I look at that list, I realize that all but one of them are imports from the original Rocksmith. Maybe RS2014 simply has more difficult songs?
There are probably a few more that are easier, but I haven't made as much progress on, due to not playing them much.