Ever since I played Rome 1 back in the day, a good Flanking maneuver has always been the best feeling to me. Just getting behind the enemy and breaking them with a good charge it's an amazing feeling to me.
Knights in Med II always made this great. But Rome II... flanking the enemy and watching as they just crumble away from the perfect flank. I've won unwinable Battles with a perfect flank in Rome II.
Even now in WH3, hitting the enemy from behind with the Jade Landers feels amazing. Getting a good flank with Bretonnian Knights on the Greenskins.
It's just the best feeling in these games.