Try to get a wrench on the body of that valve, another on the bonnet nut.
Hit that bonnet nut with WD-40 or penetrating oil before removing the nut.
If the body twists, you'll be spending close to $600 to fix it.
If you have a big enough set of jaws, you can depress that nut where it changes its shape and breaks the calcium bond but it takes a good grip to do it.
Sometimes turning it slightly forward THEN backwards to remove can help remove it.
Remember, 2 wrenches to take that bonnet nut off; one on the body to steady it and one on the bonnet nut to remove.