The Serengeti and Etosha are both great, and both quite different. I saw a lot more predators in the Serengeti -- I know this is partly luck but if you spend enough time in the Serengeti you can realistically see lions, leopards, cheetahs and spotted hyenas. The wildebeest migration is spectacular. Hippos and crocodiles along the river. There is always something to see.
I loved the desert landscape of Etosha and its large herds of different antelopes, plus wonderful elephant viewing.
Ngorogongoro Crater is actually my least favorite of the northern Tanzania parks. Yes, you can see plenty of wildlife up close, but it is also extreme.ly crowded and somewhat small. It is worth visiting, to be sure, but for me it does not come close to the wildness of the Serengeti.
Tarangire, to my mind, is better than Ngorongoro. The number of elephants alone make it worth the visit. I found it a beautiful park. You can also get way off the beaten path there...few visit the southern part of the park.