I went on safari to Kenya and it was truly fantastic - go to the Masai Mara - and hopefully your tour guide will venture into the serengeti in Tanzania! The Masai Mara is the biggest and the best park. The serengeti is the next biggest (I think) and the landscape, although beautiful, was not as breathtaking.
DO stay in a luxury lodge for at least 2 days...Not too expensive and everything is usually included and you are guaranteed to see at least the 'big 4' out of 5.
If you decide that you want a more rustic slash back to nature experience, then look around for a guidebook recommended lodge/camping site, as a few friends of mine went for the cheapest, and saw very little as the guides were not as experienced!
Updates
I did a five week overland tour from Nairobi to Jo'burg recently and the Masai Mara was by fair the best reserve we went to. We were so lucky in seeing four out of five of the Big Five in the space of two days - it was awesome.
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