There's a lot of factors. I think Texas would be rolled over pretty easily. It would be one of the tougher non-insane states. I think it's got to be Michigan or Minnesota though. Michigan has the most licensed hunters of any state in the US. Yes more than Texas. SO that's a lot of people who're used to hiding and shooting things from afar. Not to mention they have a whole city chock full of hiding spots since no ones in it.
Minnesota is a miserable place. I think invading Minnesota would be like a mini-march on Russia. Alaska aside from the getting there would be easy. Invade during the summer, almost all of the people like in a relatively small area. Hardest part about them and Hawaii is access. With Alaska you'd have to move from Washington, Oregon or California, or have Canada let you cross on land which I don't know if they would.
A state no one will probably mention is Pennsylvania. There are a few reasons why it would suck to be in a fight with pennsylvania. For starters, it's second or 3rd on registered hunters, they take that **** seriously there. Second it's a large state for the north east. YOu may be used to fast and furious fighting in NJ or Connecticut, then you move on to pennsylvania and it's a snooze fest.