Russia because of the cold.
And the U.S because of the fact it's in the middle of nowhere and every 'citizen' is going to have a bloody gun, that and the stupidly high number of nukes.
Invader: "We don't like you, we're invading you U.S!"
U.S.A: "EAT FREEDOM *****!" *turns rival country into radioactive dust*
Fixes to invade these country's easier: Russia: Give everyone woolen hats which look awesome. So nobody freezes to death.
Fixes to invade U.S: Don't invade, nuke them first, take out as many silos as you can - and be prepared to shoot down any they launch. That and when you DO need to invade either: Have a VERY big bridge built, a hells big fleet or be Canada/Mexico.
So there you go, the hardest to invade - and the solutions
