I have booked two private tours of four days each with Priora Tours. I have found the company to be very easy to work with and they provided all documentation required for my initial visa. Their tours included a bi-lingual tour guide, a driver (when required), all admissions and helpful local knowledge of the area. I find the private tours provide a significantly better experience, because you can spend more time on things you like, less on others and rearrange schedules as required. Because the guide was only handling the two of us, it was easy to ask for more details and they learn what you are most interested in, so that they can point it out for you.
I enjoyed the experience so much with my first tour using Priora Tours to St. Petersburg in 2015, that they were the only company I considered when I planned a follow up trip to Moscow. I felt that they provided the white glove service from the time they picked me up at the airport to when they dropped me off for my flight. Even assisting with local suggestions for days I had no tour scheduled.
I would definitely both recommend them and use them again for a future trip.