We did a 15 day trip to Tibet with Alpine Eco trek from Kathmandu going through Lhasa, Everest Base Camp and the Mt Kaialsh Kora. It was an amazing experience and very ably handled by our guides who had to think on their feet when a snowstorm hit and the Chinese authorities diverted us.
Tibet itself is another world but becoming much easier to travel through thanks to the new roads the Chinese are bringing in and I couldn't recommend enough making the effort to get out west to Mt Kailash. Once back in Nepal Alpine Eco Trek also handled buying our bus tickets and finding our porter so that we could do the Annapurna circuit for no extra charge than what the expenses were.
If you want great, friendly yet professional service then go see Ram and his team in Kathmandu, they'll make your trip to the Himalayas even more special!