Maine only has to defend one border.
Of the 50, Hawaii or Alaska, hands down.
If we’re talking about the contiguous 48, that’s a tougher one. Probably one of the west coast states or Maine, due to the land approaches being covered in vast mountainous forests and having ocean on the other side. The west coast also has a huge series of mountain ranges to contend with.
Alaska could just do Russia’s ww2 tactic
Strictly geographically, either Washington, Texas, or California. They all have major ports that they could use to get weapons from other countries as well as being out of the way.
Texas might have the added advantage of being able to illegally trade with the drug cartels of Mexico for weapons. Other states can have land or sea contact with Mexico, but Texas has the biggest reach out of them all.