We recently had the pleasure of traveling through Tibet, Nepal, and Bhutan with Wind Horse Tours. They offer a number of different options and have expert guides in each country. They worked very hard to develop an itinerary that worked for everyone in the group (6 of us total) and made adjustments on the fly as necessary. This is a fairly difficult part of the world to navigate and they really did do a nice job.
The accommodations throughout the trip were good to excellent and our guides did a nice job of calling in advance to see that things were tailored to our needs (2 veg and 2 no spice). This was particularly true in Bhutan and we felt that we were treated extremely well. I would recommend Wind Horse without any reservations.