We booked with Monkey a Safari around Arusha (Serengeti, Ngorongoro, Tarangire) and we are very happy with this choice. The service was perfect and we've got all the help we needed during the trip. David, our guide, is a very nice guy with a very good knowledge about wildlife. We have lernt a lot from him! Also he was very good at finding wild beasts. I can definitively recommend Monkey Adventures.
If you plan a shorter Safari, I suggest to skip Tarangire. You can see the big five in Serengeti and Ngorongoro.