The battle with Gortheron, especially Gandalf's Big Damn Heroes moment at the end.
Slaying Draigoch, a full-grown live dragon. (Also Thorog, a full-grown undead dragon).
Defying Saruman, breaking his Ring and getting away with it.
A certain Scavenger Hunt quest ups the ante - you must call him crazy to his face before the fight.
The session play "We Cannot Get Out", depicting the last stand of the dwarves of Balin's Company inside the Chamber of Mazarbul.
Rousing the Ents at the culmination of first the Eaves of Fangorn quests and then the Balewood quests.
On These Stones, in which you unite with various leaders in Rohan to take down a Black Númenórean, similar to Mordrambor from Volume I of the Epic Story.
The first time a Nazgûl is fought and defeated, in Epic Volume I Book 4.
Epic Book 5, which takes place in the Misty Mountains. First, you slay a goblin leader, then you storm a Dourhand stronghold and slay their leader, and finally, you head into the Helegrod Treasury to face the Nazgûl again, and stop him and his allies from controlling a Dracolich. You only manage to banish the Nazgûl back to Mordor, and temporarily keep the dragon out of the enemy's hands, but WOW.
The two confrontations against Mordirith in the epic, first in Book 8 then Book 15.
Storming Mazog's throne room and taking him captive in Epic Volume II Book 7.
Storming Barad Guldur and killing Mazog at the end of Epic Volume II Book 9.
Helping Aragorn retake Pelargir and kill the leader of the Corsairs of Umbar in Epic Volume IV Book 2.