The best achievement of his first term was complete but incospicious victory in Libya. The country which gave up nuclear weapons and rejoined family of law abidding states without single shot being fired.
The worst one? The Abu Ghraib Iraqi prisoner abuse scandal ... no need to continue.
The biggest challenge of his second term is probably Palestine situation. It appears to hold a key to many problems of the current world.
It is a pity that Americans do not stress more often that they do not fight against Islam - their arms saved Muslims against Christians in Bosnia and Kosovo after all.