Biggest jump in quality?

What’s the biggest jump in a band’s quality you can think of from one album to the next?

Here are the top 5 I can think of in prog rock:

5. Pink Floyd: Obscured by Clouds to Dark Side of the Moon
   Obscured is a good listen, but wow Dark Side is a big leap forward

6. Yes: Time and a Word to The Yes Album
   Time and a Word is good, but they entered their prime in the next.

7. Jethro Tull: This Was to Stand Up
   This Was is a fun bluesy album, but showed not a hint of the classic that followed it

8. Jethro Tull: Under Wraps to Crest of a Knave
   Big rebound after a clunker

9. Genesis: From Genesis to Revelation to Trespass
   You can get whiplash from the move from amateur hour to prog gods.