Mine has to be the epic siege of athens (Total Realism mod, nor/nor). 2 Roman armies had landed and besieged athens. They attacked with 2 sets of ladders, a ram and over 5,000 men (Huge units). I had about 900 men. 1 general, 3 archers, and 4 athenain hoplite(same as normal hoplite, just with different skin).

The assualt begins. I put my archers on the walls with 3 units of hoplites. I left one in reseve with my general. The romans' 2 armies start on opposite sides of the wall. The northen army has the ladders and ram, but is smaller. You can guess what. The ram on fire. Two units coming up each ladder then getting overwhelmed to the top. Every one else stands around the gate and gets killed. The other army marches around next to the wall and dies.

They tried it again a few years later and the same happened. Then they seiged athens for the third time, but then with most of their forces away, I stuck at them and took their last 2 cities in one turn. Their armies around athens turned to rebels and I bribed them. Now one of them is the governer of athens.