I think the majority of them are simple if you can play the full major and minor bar chords as Frank LOVES to use em.
Easiest ones IMO for a beginner:
Holiday Song Nimrods Son Cactus Where Is My Mind Gouge Away
Old Black Dawning Brackish Boy Calistan Headache You Ain't Me Jesus Was Right Doggone Do You Feel Bad About It 666 Tiny Heart Think Im Starting To Lose It Llano Del Rio Chip Away Boy 21 Reasons
All of the above are fairly basic...sometimes he gets a tab bit freaky with the multiple chord changes (and he's a genius at changing octaves and going right from major to minor...he even makes point of it in the song "Bullet")....Kingly Cave has some crazy chording and songs like "Swimmer" go major to minor all at the drop of a hat. But really, overall, his songs are quite easy.