The Northeast covers very different landscapes, so there is no single best month for every state. Use shoulder seasons for comfortable sightseeing and always check live weather and road advisories before driving into hill regions.
Assam
October to April is a common planning window for wildlife and city circuits.
Meghalaya
October to May is often easier for sightseeing, while monsoon months can bring heavy rain and dramatic waterfalls.
Sikkim & Arunachal Pradesh
Altitude, snowfall, permits and road conditions can change the practical travel window. Build a little flexibility into the itinerary.