Which of the following anaesthetic produces best uterine relaxation?
Halothane is the anaesthesia of choice for internal version and manual removal of placenta.
Halothane 
Pharyngeal and laryngeal reflexes are abolished early and coughing is suppressed, while bronchi dilate. As such, halothane is the preferred anaesthetic for asthmatics. It inhibits intestinal and uterine contractions. This property is utilized for facilitating external or internal version during late pregnancy. However, its use during labour can prolong delivery and increase postpartum blood loss. 
Reference: Essentials of Medical Pharmacology Eighth Edition KD  TRIPATHI  page no 406
Nitrous oxide
Halothane
Chloroform
Ether