The brass FPT to flare fitting on the right side of the valve?
I would use either a large crescent wrench or a pipe wrench on the valve (it won't fit perfect, but the idea is to keep the valve from moving), and an open end (preferred) or crescent on the fitting, loosen - if it won't budge, try tightening then loosen, if it still won't budge heat it with a blow dryer or heat gun (but not a torch for obvious reasons) and try again.
Before the days of teflon tape some pipe joint compounds were more like epoxy as they got older and almost impossible to move.
Longer wrench handles, more leverage, easier to move.
Never use anything on flare joints.