Lots of super recommendations here - clearly ladies of taste! Cragside is gorgeous, Lanhydrock and Cotehele ooze tranquility, while Castle Drogo is on my bucket list for a third visit, now the works are completed.
Shugborough was the scene of an early date with now-DW Ashcombe, and her friend worked there.
I come from the SouthEast, so must nominate two open spaces there - Box Hill and Leith Hill, the latter the highest point in Southern England at 960 ft, both NT-maintained. Gorgeous views over the Weald to the South Downs.
Finally, in Kent, Sissinghurst Castle Garden, which Vita Sackville West created, and I think is the most visited of all the NT gardens. Proud boast - late first wife and I delivered a load of rotted horse manure to The Gardener there - whereupon she asked for more!