A travel insurance policy that covers theft, loss and medical problems is best for traveling in Nepal. There are a wide variety of policies available, so check the small print carefully. Make sure are covered for adventure activities and high altitude.
Since you reach above 4000 meters on quite a few treks in Nepal, it is best to choose a policy that covers medical and emergency repatriation, including helicopter evacuation for trekkers and general medical evacuation. Also, understand that most medical treatment and facilities/hospitals must be paid for at the point of delivery of the patient. So it’s wise to choose a policy that pays doctors or hospitals directly rather than you having to pay on the spot and claim later.