We are Road Scholar fans. They provide more value-added than tour companies who provide buses, one guide for 25 people, and entire buffets with nothing but dull food. Prices usually represent good values too.
That said, we took an Ethiopia trip with Nat Geo. It was by far the best organized and best run group trip we have ever taken. It was wildly expensive though. Definitely not a good value. We were there for a week with Nat Geo, then another week with a private guide and driver. That second week cost about half what Nat Geo charged for their group travel week.
Don't overlook the option of hiring private guides. They are expensive in Europe so we tend to not hire one every day. But a day with a private guide is a great introduction to a city and the guide can help you plan the rest of your days in that city. We do this in countries that use the Roman alphabet so we can read the street signs. Non-Roman countries (Arabic, for example) we always have private guides and make travel arrangements with a local company. We stay in the kind of smaller hotels and BnBs we like, we eat at excellent local restaurants, see what we want to see, and do it all on our own schedule. We never wait for the ladies who shop beyond the bus departure deadline or who have to go to the loo after the deadline. We specify that we want older guides with the life experience to really teach us about their country, not the enthusiastic kids who only know the dates of all the statues, paintings and historical events.