Zeppelin is the most overrated band in history. They were a great jam band, but had one sound and never evolved.
I'm going to have to disagree rather strongly. Zeppelin evolved quite a bit and and I would say more than the Stones.
Both bands started as heavily blues-based. The Stones became more of a straight ahead rock band while Zep continued to change until the end.
Zep's first album is mostly a combo of blues songs and in your face hard rock and the songs were mostly "borrowed" from other bands. Second album is much the same except now they have written all of the songs themselves. By the time they get to IV, they are playing Stairway to Heaven, which we all may be sick of, but it is way, way more complex than anything they had done (and way more complex than anything the Stone ever did). They are now using maj 7 chords, min/maj7 chords, 9th chords and the whole song builds in a way that few, if any, rock songs have before.
Four Sticks - played in a mix of 5/8 and 6/8 time - this is jazz type stuff. The Stones never played any odd time signatures - hardly any even in 3/4 or 6/8.
Rain Song - again, an array of complex chords - maj7, sus4, m9, chord inversions. Heavy dynamic shifts, going from soft acoustic to intense orchestral (actually a Melotron). This song would be so out of place on their 1st album.
Kashmir, Ten Years Gone, Achilles Last Stand. They sound more like a prog rock band in the mid 70s than the blues based band they started as. Started using synth in a number of songs. Listen to In Through the Out Door - it is so very far from Zep I.
The Stones evolved quite a bit up until Beggars Banquet, when they hit their stride. To me, they don't evolve a ton after that. Not necessarily a terrible thing. They found exactly the sound they wanted and stuck it hard. Maybe better to do that than risk evolving into something that doesn't sound that good anymore.