Most potent antiemetic agent used in preoperative period –
Commonly used antiemetics in pre-operative period are Hyoscine, Ondansetron, Perinorm.

Amongst this Hyoscine is most potent :-
"Hyoscine is the most potent antiemetic to be used as part of premedication".      
— Fundamental of anaesthesia
Glycopyrolate
Hyoscine
Atropine
Metochlorpromide