just some extra info... I have lived in Kenya my whole life, I've been to numerous national parks including lake manyara national park and ngorongoro crater in tanzania. Tanzania, is much more expensive than kenyan parks, and my favourite would have to be masai mara and lake nakuru national parks in Kenya. Masai mara is great for all wildlife in general, however I have been there atleast 7-8times (with 2 game drives a day, for atleast 2 days) with professional guides and only saw a leopard, twice and rhinos are difficult to spot as well, but I can guarantee you would see many lions, elephants, buffalo, giraffe, birds, and a large variety of land herbivores. Lake nakuru national park is great for seeing leopards, the huge flamingo population (which has actually decreased dramatically now - but still a beautiful site when I went in summer of 2006) and white rhinos.
Hope that helped, and if you have any more questions, feel free to ask!