The Big 5
If you must see all of the big 5 on safari, that very much determines where you must visit, as Rhino are only visible in the Ngorongoro Crater - on the Northern Circuit.
All other members of the big 5 can be seen in many parks so if Rhino isn't a "must" then you have the option of many more parks both in the north and south of the country.
It's common for visitors to want to see the big 5 on their first safari, then the next time, they are free to visit some of the quieter parks. Seeing the Big 5 isn't guaranteed - all but Rhino is almost certain, but there are few rhino in the crater and the crater is large. Often they are seen at a distance, but sometimes guests are lucky and get to see one up close.