My biggest victory was in Rome: Total War - Barbarian Invasion and I was playing as the Eastern Roman Empire and one of my cities was besieged by the entire Hun population (3 armies and they were attacking from different directions) and my garrision cosisted of 3 archers and 4 units of basic infantry and 2 heavy infantry. I managed to win the battle thanks to the careful deployment of my men and also down to the fact their armies had large numbers of cavalry units.
My largest battle, was on the original Medieval: Total War. I was the Byzantine Empire and I bankrupted my Empire building an Army of 45,000 to fight the Mongol Invasion. It went wrong when the Mongols turned up with 120,000 men. Needless to say I didn't win and my empire collapsed, because my entire royal family died on the battlefield.