Probably a three way tie between Hetch Hetchy Valley Restoration, Owens lake, and ancient Lake Tulare.
So much of California's beauty was wrecked from the 1880s through the 1960s.
Hetch Hetchy was thought to rival that of the sister valley - Yosemite. Nobody in their right mind would build a dam in Yosemite - but Hetch Hetchy got wrecked for SF water supply.
Owens lake / valley was robbed of it's water by LA back in the early 1900s. Place is a wasteland now. (Yes robbed - they lied about the intent of buying the land then sucked the valley dry)
Tulare lake basin was a large seasonal lake that would form yearly in the central valley - after everything was dammed the lake was no more.
By freeing Hetch Hetchy of the dam and expanding the national park area out from Yosimite - California could have an amazing Sierra Nevada national park with wetlands and valleys - could even reintroduce grizzlies and wolves - but alas, never more.
Hetch Hetchy Before Dam:
https://www.sacbee.com/opinion/article80240957.html 
