the Kael'thas fight (my favorite fight in the game)
Kael was indeed a lot of fun, but I think I prefer Vashj.
As for weapons, in my mind, it's my characters who have favorites. Poldaran loves Quel'delar
. Azmaria is a huge fan of Armageddon
. Alexandrine loves the Arcanite Reaper
. And Sindara just likes wielding a staff