Most potent bronchodilator among inhalational anethestic agents is-
(Refer: Morgan and Mikhail’s Clinical Anaesthesiology, 5th edition, pg no. 167 – 169)
Effect of inhalational agents on respiratory system

All inhalational agents are bronchodilators →Halothane cause maximum bronchodilatation.  Inhalational agent of choice in asthmatics (intravenous anaesthtic of choice is ketamine).
All inhalational agents cause respiratory depression to some extent → Maximum depression of respiration is seen with enflurane, on the other hand maximum inhibition of ventilator response to increased CO2 and hypoxia is caused by halothane
All inhalational agents vasodilate pulmonary vascular bed by blunting the hypoxic pulmonary vasoconstriction (HPV) response.  Halothane has maximum effect: Isoflurane, Enflurane, desflurane, sevoflurane have similar effect.
Isoflurane
Sevoflurane
Halothane
Desflurane