We booked Everest base camp heli trek with Highland Expeditions in late Nov, 2017 and it was an experience of a lifetime. 
The trek to the base camp of Everest is itself an amazing experience beside we took helicopter flight back to Kathmandu from Pheriche which allowed us to see the mighty mountains from different angle but what made the trek even more amazing is because of our trekking team (Guide Kunga and Kancha) and expertise showed by entire Highland expeditions team. Passang always answered our email promptly and gave us detailed information about trek, and after we landed in Kathmandu, everything was delivered as promised. 
I would highly recommend Highland Expeditions to anyone looking for trekking and climbing operator in Nepal as you wouldn’t find any better than them.