That's a tough one...lots of old survivors out there...
I'll bet BeeeeG bottom is a candidate but no trails in the area even historically.
Bagby area, and south of there too. And down where th Willamette NF starts.
Even up at Memaloose. It hard to say, I've never measured individual trees but it would make an enteresting journal.
I'll betcha the biggest are down in river bottoms, cool and dark. SPOOKY.