I'd like to start out by saying that there is no nation on earth that can effectively counter ANY invasion. ANY nation on earth could theoreticlly be humbled by an invader with the technology, tactics, and numbers to accomplish the goal. There's no such thing as an unbeatable nation, no such thing as an unbeatable armed force.
That being said, I'd like to point out that Germany has a very long tradition of having a fine military, and a particularly fine army. This tradition continues today.
I don't believe that any other army in the would could best the Bundeswehr on German soil without the use of nukes. The US might give the Bundeswehr a run for it's money if they could come through France or the low countries, and maybe Russia could if it ever got back into steamroller mode ala WWII or cold war, but I believe both of those possibilities to be remote in the extreme.
As far as power projection goes, Germany isn't quite so strong. She doesn't have the navy or airfleet to deliver or sustain large numbers of troops abroad. If Germany had overseas possessions like Britian, there would be no successful Falklands in Germany's future, given current equipment. She can project power as far as her tanks can secure airstrips. France, Poland, Czechoslovakia? Still not a problem. South Africa, Peru, Bangladesh? Not happening anytime soon.
Cheers