On default MTWVI there are two hilly green maps I've seen several times in the Balkans and other parts of Europe which I assess as the best defensive maps simply because there are two of them, I can reasonably expect one or the other to show up in many critical battles in certain provinces. One of them involves a sharp hill overlooking low ground faced with cliffs and which I usually use defending against Poles or Hungarians. It is extremely difficult for the AI to assault and the only time they've ever come close was one battle against the Swiss in central Europe.

The other is a semi-forested map with a steep hill near the edge and in many cases, the AI seems to have trouble understanding how to flank you. I've parked a dozen units of arrow on there and conducted some terrific massacres.

The third is an extreme mountain map I've encountered maybe twice in Georgia. Sometimes I send an army there in the hopes of getting this map as it can be possible to hold the first wave of the Horde with a buildup starting in High.