There is no single best tour, and any page claiming one is guessing. What there is: a best trip for the number of days you have. One day does Carthage and Sidi Bou Saïd properly. Two to three reaches the Sahara. A week takes in the Roman cities and the desert. Fifteen days reaches 9 UNESCO sites — every one open to visitors. All 28 are private, priced per vehicle from €74.
Judge on three checkable things rather than on adjectives: is the price per vehicle or per person, is the vehicle genuinely yours alone, and is the full price published before you enquire. An operator who will not put a number on a page before you contact them is quoting you individually for a reason.
Less than most people expect, because the price is per vehicle. A day tour starts at EUR 74 for the whole car, so four people pay under EUR 20 each.
At minimum the vehicle, the driver, fuel, tolls and parking, with hotel pick-up and drop-off. Site entry fees and lunch are normally separate, and on multi-day trips accommodation is normally separate too — check that before comparing two prices.
Distances are straight-line, measured between the coordinates on our map of the sites; road distances are longer. Prices are per vehicle and come from the same database the 28 tours and 1,392 transfer fares are quoted from.