If we count the whole Denerim section of Dragon Age: Origins, then that. It's huge, dangerous, and you get to see the culmination of all your work through the game pay off as you call in various allied forces to help fight through an unbelievable number of Darkspawn, in now-ruined and burning streets that you've spent hours questing in earlier. Then, finally, you reach the top of Fort Drakon and face the Archdemon, where other named characters from through the game will also come and fight with you. It's an epic (to use the word properly!) and deeply satisfying climax to an enormous adventure.

dragon-age-origins35.jpg

Agreed with WK about Caer Xhan/Orbital Station Myria. Deeply atmospheric and enjoyable to explore.

The final assault on Red Mountain in Morrowind. You've seen the volcano from afar across much of the island, looming as an ever-present reminder of danger. You've ventured beyond the Ghostgate previously, and armed yourself with the relics of those missions, but now you seek to enter Dagoth Ur's own base. It's a pretty brutal battle through the depths of an ancient civilization's ruins, to destroy the Doom Drum.

8V114tE.jpg

That's all I can bring to mind right now, but maybe I'll be back with a couple more later!