Going to sleep at the Bloated Float, the ship repurposed as an inn at the imperial waterfront. Waking up to find out it had been hijacked by bandits and was heading out to sea. Sneaking through the ship to take down the bandits one by one, before confronting their leader on the deck, under a full moon, in the middle of the ocean, and drawing our swords to dance. That's when I knew Oblivion wasn't like other games.
Luring enemies across trip wires and false floors to turn the physics-based traps of the dungeons against my foes.
Walking through the forests overlooking the Niben river, with sunlight spilling through the branches, and coming across an abandoned chapel. There were signs someone had been through there recently, possibly looting the site for materials. When I entered the chapel, there were two necromancers performing experiments on a cadaver on the altar. They stopped what they were doing, looked over their shoulders at me in the doorway, then drew their weapons and summoned zombies to take me out before I could report their activities to the authorities. As I played the game, I kept hearing about what a big deal it was that the Mages Guild had outlawed Necromancy and its practitioners must now operate in secret.
In the Red Wastes of Oblivion, raising bridges between different guard towers so I could eventually activate the bridge that led to the central tower that controlled the Oblivion Gate. Walking hundreds of feet in the air between these towers, I realized this game is actually top 5 in terms of dungeon design in my experience.
Looking for a woman's missing husband and her debtor taking me to an abandoned island fortress, where they charge criminals and murderers to hunt other human beings for sport. I escaped by turning the game against them, jumping unseen among the rafters above them in stealth like Batman and jumping down to take them out. When I made it out, after failing to save her husband, I made it back to Bravil in the middle of the night and woke her up to inform her her that he was dead. She gave me a book about the War for the Red Diamond. I read it cover to cover and never sold it.
I played Morrowind after finishing Oblivion. I liked the story and setting but it was not a good game. Even the act of moving your character was stiff and awkward, to say nothing of having to stop running every forty feet so you can regain your stamina, so your attacks won't consistently miss against the next cliff racer or slaughter fish which is assuredly coming your way within the next thirty seconds. After Morrowind, I returned to play Shivering Isles and I remember just jumping around for a bit, so happy to be back in Oblivion where the movement is so fluid and fun.
The one thing that really stands out to me about Oblivion is the emotions it conveys about the joy of solitude in nature. Bethesda achieved this in part by generating it's world with an algorithm to have natural topography. This made the valleys stand out, how you can't really see what's at the bottom of one until you make it below the treeline. Everything just felt so real and natural to explore, it enhances the curiosity of exploration when you can't see what's over the next hill. Skyrim is pretty flat in between it's mountains and Morrowind's hills are the size of sand dunes. It's the use of topography and elevation that make Oblivion my favorite Elder Scrolls to play and explore.