Every single moment was a highlight, but some of the best things were:
Lions that crossed the road right in front of our jeep, and then sauntered down the hill and climbed a tree.
A visit to a Masai town where we watched dancing and were invited to sit in the chief's wattle-and-daub hut.
The first glimpse of Ngorongoro Crater, the world's largest intact caldera, teeming with wildlife (the elephants are just visible from the rim).
A blood-red sunrise over the dusty Serengeti.
The thousands upon thousands of birds on Lake Manyara: flamingos, pelicans, egrets, and the fantastically ugly marabou cranes. If you like birds, don't miss Lake Manyara.
Animals everywhere! Lions, cheetahs, baboons, zebras, elephants, buffalo, crocodiles, hippos... thousands of them!
Luxurious game lodges where the monkeys patter across the roof like Santa's reindeer and buffalo occasionally stray onto the lawn.
Our guide, Yusuf, who was remarkably knowledgeable and tactful: he was just as good at taking care of us as he was at pointing out wildlife.${QuickSuggestions} Learn some Swahili! It's a surprisingly easy language.

Find a reputable safari agency; it can save you a lot of legwork. You don't save much money by going solo, and you need a guide to get into the biggest parks anyway, so there's no reason not to. But don't go with a disreputable one!

Bring a sweater and warm socks. The elevation is high, and it can get COLD.

Buy your drinks (water, beer, wine, soda) at stores in town instead of at the hotel -- it's cheaper, and you'll be able to keep them in the jeep cooler. (Though don't go getting drunk on safari! That would be silly.)

Bargain hard for everything.

When you find something you want to buy, go ahead and buy it. Shopping opportunities are pretty thin in the Serengeti. If you're spending any time in Dar, you'll have a chance to buy anything you missed, but if you're just out on the safari circuits, there isn't much point in holding off. (Unless you think you're being overcharged, of course.)

${BestWay} Hire a good four-wheel-drive jeep and, preferably, a driver-guide. They know how to find the animals, and unless you've already been trained in this habitat, you probably don't. Our jeep seated six people comfortably. While only five seats are window seats, that's OK, because the roof opens, and when you're standing, everyone has a good view.

There are some larger buses that carry big groups into the parks. They looked like a bad idea to me: too big for a lot of the roads, and since they didn't have open tops, passengers had to crowd together at whatever side was looking at the animals. I wouldn't take the bus if I had another alternative.

There are good maps available at the large souvenir store just south of Arusha.

From journal A Safari in Tanzania