Hey Jordan!
For markets that I am familiar with - I would recommend the following:
Manchester, Nashua, Concord, Dover, Somersworth, Rochester
These towns are all larger multi-family communities with a decent amount of multifamily inventory and a good draw. NH rental supply is one of the lowest in the US. These areas benefit from good overall cashflow and moderate to slower appreciation.
For similar towns in Southern Maine:
Sanford, Biddeford, Saco
If you wanted to consider more of an appreciation market or class A type of rentals:
Portsmouth, Rye, Hampton, Seabrook, Newmarket, Durham, York, Kittery, Portland.
I frequently work with a lot of investors that are doing exactly what you are. Within just about an hour or two of Massachussets there are a lot of great rental markets with low supply, extremely high demand and appealing cashflow. Nh from my experience is also a bit more landlord friendly than Maine and Massachussets. Happy to help if I can.