All of the following are obligate coronary vasodilators except:
Nitroglycerine
Nitric oxide
Hypoxia and Hypercapnia
Acetylcholine
All are known to cause vasodilatation.
Only acetylcholine can also be a vasoconstrictor if the endothelium is dysfunctional or absent and nitric oxide can not be produced. In that case it is not an obligate vasodilator.