The Serengeti is perhaps the most famous national park in Africa - host to one half of the annual wildebeest migration that sees 1.5 million of these rather ungainly herbivores travel between Kenya's Masai Mara and Tanzania's Serengeti to the south. The Serengeti is the larger of the two reserves and despite the huge volumes of tourists that come through here each year, it's slightly easier to shake off the minivans than it is on the Kenyan side of the border, especially in the northern reaches of the park.
As with the Mara, it has to be seen to be believed when it comes to the sheer volume of plains game - especially in peak season, when the herds head south. For anyone planning their first safari, I would recommend northern Tanzania as the single best destination on the continent if you want to see everything. A trip here isn't limited to the Serengeti - you can easily take in Lake Manyara, Tarangire and Ngorongoro Crater in the same visit, all of which I rate very highly.
Of these, Ngorongoro Crater is particularly special - a huge natural volcanic caldera that effectively forms its own ecosystem. The density of fauna here is extraordinarily high - and while some zebra and wildebeest do emigrate in the rainy season, the majority of the 25,000 or so large animal species in the crater are resident year-round, making it an exceptionally reliable photographic destination.