Beating Firkraag, beating Demogorgon, beating a certain lich whose location I can't remember now. In fact there is so much I've forgotten ...
One that really sticks in my mind for some reason, though, is a battle in the lower regions of Watcher's Keep (and I LOVE the music which plays when you enter the Watcher's Keep area btw) : the enemy party consists of six members, each one very different from the others, so it's 6v6 and it is TOUGH. The loot you gain when you finally win the battle makes the effort worthwhile though.
Edwin changing sex.
Just so many memories, most of which I've forgotten, if that makes sense ?