Northeast as a whole is worth visiting.. I will suggest visit one state at a time.. Each state has something unique to offer .. I have been to Meghalaya and Assam.. Rest yet to go..
All the north east states are worth visiting.But if I had to name any one then Meghalaya is splindid place to visit. you can plan a 6days itenary for yourself.
Hey I'm from North East
Surely, I would love to help you out in this. Almost every state's of North East is best to visit but I have visited these state's so I'm suggesting you -
Assam: Guwahati, Sivsagar, Tezpur
Gangtok Meghalaya : Shillong, Jowai, Dawki, Cherrapunji
Arunachal Pradesh : Ziro Valley, Mechuka, Roing, Pasighat.
Nagaland : Dzukou Valley , Dimapur