Serengeti (and Ngorongoro Conservation Area - outside the Crater) are going to see millions of wildebeeste and zebra calving in a short period of time. The congregation of animals and the increase of carnivores make this of itnerest. - February.
These same wildebeeste travel in a clockwise circuit throughout the year, and by July or September are in the far North of Serengeti at the Kenya border. Here you may see the river crossings that National Geographic makes so exciting.
Manyara is good any time of year. It is a green wooded park - different from the others. Some like it, some find it not as exciting.
Tarangire has a mini elephant migration between June and November, making it a better time of year for that park.
Not just ellies in Tarangire. 250,000 Wildebeeste and other ungulates pour in from the Maasai steppes and Kilimanjaro area for the permanent river from June-December's little rains.
Tall grass in Ngorongoro can make viewing tiresome up to about June so I would suggest the 'best' for all round viewing is the second half of the year. The exception being Ndutu area in February-March.
Its better to visit there any time of the year but sometimes its not good on march, April and November it rainy during that time the roads are not in a good condition it may be a trouble to you though it is not guaranteed.
OP are you still there I presume you already have decided when you are travelling.
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