Selous Game Reserve offers expect knowledge of the area and the animals that inhabit it. The actual area is roughly 52,000 square kilometers so it is never guaranteed you will see animals. However, the rangers know enough to take you to the best spots.
They also talk to the other rangers in case others have seen animals you haven't.
The only bad aspect is they tend to drive quite fast at points and you can get thrown about a bit in the Land Rovers. Expect a few bruises when you finish.