easiest/basic chords: All I Want is You
3 chords - slow tempo, it doesn't get any easier.
Now the delay on the electric - to play the full blown version - is tougher but for basic chord version that's about as easy as it gets.
At least other songs have a faster tempo. And this is usually the answer to the question from a beginner: what song should I learn first. I'd be hard pressed to suggest a better starting point.
Remarkably, most of the more recent stuff is easy to play as well. I could play all of Vertigo after the 2nd or 3rd time I heard it. But a beginner would have trouble with the tempo. However, if you can use power chords or barre chords, that song is simple as shit.
hardest: Wire
I can play them all - pretty much, including a version of Mofo. But Wire, for some reason, is probably the toughest for me to play. Edge is such a master of the delay and he used harmonics so wonderfully back then, I just can't ever quite get that song right.
Comparatively, Bullet isn't that hard, for me because I grew up playing metal and shit like that. And I would suspect, that if you asked most seasoned guitarists that have played a lot of Zeppelin and in that sort of vein, that the songs like Wire would be the toughest because of the delay, not just the setting but literally Edge's style of picking, harmonics etc.