Unique is a thoroughly overused word, but it really does apply to the Ngorongoro Crater. This is the largest unbroken caldera in the world - and it's full of big wild animals. The difficulty here is remembering that what you're looking at isn't artificial. And this becomes harder when you realise the only thing to rival the animal population in Ngorongoro is the tourist population...and for this you must be prepared. On a bad day you'll can find yourself persued by minibuses full of camera-toting tourists each time you stop. It's probably harsh to suggest that you should miss the Crater out altogether on account of the traffic jams, but our advice would be plan to leave wishing you'd stayed longer, rather than the other way round.
Tourists or no tourists the Crater itself is one of those sights you really have to see to believe; formed with the collapse of a massive volcano (quite possibly rivalling Kilimanjaro in height) it's been preserved as a perfect bowl some 18 km across. The ground area is just over 260 square kilometres and within this relatively small space most of the major east African habitats and mammal species are represented. What's more you have an incredibly good chance of seeing them here on your safari.
The Crater is over a third of a mile deep and the scale and perfection of the thing is staggering. Once on the Crater floor, most of the animals at Ngorongoro, whilst totally wild, are very used to vehicles. This means that they all but ignore them (which at times must be very hard to do) and as a result they can be approached fairly easily. This makes the Ngorongoro Crater an ideal stop on any safari and an excellent place to take children as intervals between animals are generally short and the game is often close enough that you won't need to look with binoculars.