I just finished up a week-long trip to Circle Pines KOA near Williams, AZ. This was a GREAT place for "base camp" for adventures into the surrounding area - ie the Grand Canyon, Flagstaff, Sedona.
The campground was very nice, and the staff was super friendly. There was a clean, indoor pool, very well-kept bathrooms with showers, a game room, nice laundry facilities, and a general store that was well stocked. The sites were very clean and well maintained. I was especially pleased with the multiple sewer locations in the site which made hooking up my two grey water tanks for dumping far easier than normal.
Here is a shot of our site in the campground:
As you can see there are many trees, rather than that open lot feel. It was indeed more of a "forest" experience than I'm used to seeing in most of the KOA's I've been to - they usually seem to cut down everything in order to maximize the number of spaces that they can fit into the location - not here!!
There was one "con" to this location - highway noise. The campground is located right off of Highway 40, and you could hear the truck traffic from the campsite. Fortunately, each night I was exhausted by my 3 year old son and would have been able to sleep in the center divider of that highway if needed. However, out of the campgrounds in the area with hookups, I would say that this is the best location. If you don't need hookups, Kaibab Lake would be the place to go in this area - a USFS campground, very pretty, right on the lake. Otherwise, I would recommend the Circle Pines KOA.