Everyone's going to say Texas. It's got huge borders to defend...against a handful of sates with no huge military presence, that are just as spread out as Texas and with just as much (none) defensible terrain. I say it comes down to whether Texas can tank rush, but the real winner will be...
Colorado. It's like Switzerland (or Afghanistan), they can always just retreat to their mountain strongholds. Plus they have lots of Air Force bases, and lets not forget...NORAD. Yeah, Colorado has a pretty good shot at controlling the nuclear arsenal, and if not that, definitely America's satellite network. Hmmm, satellite surveillance plus air power...I think it would be a short war.
I'm from Pennsylvania, and while I think we could do allright, we're between New York and DC. Amish country would look like the Somme, circa WW1.