This is a very general question.

I would say the 'best' defensive army are the Germans - because being neighbour to a key Warsaw Pact country for 40 years tends to focus doctrine on defending yourself (not to mention the German constitution).

The UK on the otherhand in an Island which a) provides a natural defence against invading forces and b) places greater importance on Naval and 'expitionary' forces (in order to defend trading routes and partners.

Therefore, I would say the UK has probably the most effective 'Expiditionary' forces as we can combine significant Amphibious Shipping with a number of highly flexible Aircraft carriers, nuclear-powered submarines armed with TLAMs and significant Air-lift capability (50x C-130s and 4x C-17s). All this is brought together through the Joint Rapid Reaction Force that enables the UK to conduct 2 medium scale campaigns (approx 15,00 men each) combining Commando, Air Assault, Mechanised and Armored Brigades as required.

Tell me which other country in the EU could do this?