The tallest tree would be Hyperion, a coastal redwood, which is in an undisclosed location in one of the Redwood National or State parks.
Coincidentally the oldest tree in the world is also in California, Methuselah in Eastern California is 4,851 years old.
https://en.wikipedia.org/wiki/Old_Tjikko
Depends how you count regrowing from the roots though. Trees can't be too much older because that's around the end of the Ice Age. ;-)
My personal preference is not for a single tall or big tree as much as a sprawling area full of many huge trees. If you're ever in the far-Northern-California area, this hike is amazing:
https://www.alltrails.com/trail/us/california/cathedral-tree...
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Researchers at the Rocky Mountain Tree-Ring Research Group then announced the age of another P. longaeva also located in the White Mountains — this one 5,062 years old
https://www.google.com/amp/s/www.livescience.com/amp/29152-o...