Robo and CAS239 are right. We've been on it dozens of times, throughout different parts of the day. The correct answer is that there is no correct answer. They're wild animals, it's pure luck. We've had morning safaris that had very little to see and morning ones that were amazing. Same with later in the day. On one single ride this summer we saw...a heard of wildebeest stampeding around (in fact, they caused us to have to stop for about 15 minutes), springbok running and leaping everywhere (in reaction to the wildebeest), giraffe walk right up to our truck, the cheetah actually get up and run (not full speed, but still running), watch elephants play (this was during the 15 minutes we were waiting for the wildebeest to move/settle down), lions roar. It was the most amazing thing ever. I don't recall exactly what time it was, but I think it was late morning/early afternoon. Then later that day, we had one that was sort of meh.
It's just luck, so go whenever you can/want.