Dragon Slayer, you're a funny man.

I'm jumping on the Muatra band-wagon. All of the Sermons get me going, but I especially enjoy the kinky bits
More seriously, stories of Shezarrines are great, and of course the whole cosmic drama with Lorkhan is appealing.
But what I like most are the mundane things. Little bits of history and culture and world-building that make TES unique to me.
I love the Khajiit's link to the moons and their institutionalized religious drug use. Khajiit and Ashlander culture, the aesthetics of Velothi design, a random stuff like "Who was Hestra and what were her accomplishments?" I find that piecing together a coherent world history out of miscellaneous mentions in various books is really fun!

The whole creation mythology and the way different races explain it. There's a core story there but I love the differences between them.

Only allowing to pick one is hard though. I also love Vivec, Dunmer culture(s), the Dwemer mystery, Altmer and their gnostic-like views, the Sload, Akavir and just basically everything that is and hopefully will always be mysteries.

The history of the Akavir is always something I'll bite after.

But its mostly the way the lore is presented in the games that grabs my attention.
History is written by the victor and that shows well in TES.

Dagoth Ur

Mythmaking comes somewhere around third or fourth...

Anything Sheogorath, even after SI.

The Tribunal and anything deadric.

The swordmeeting, the battle for Ald'ruhn, stuff about Seht in general, stuff about Vehk in general, stuff about Redoran in general. Oh, and Shor son of Shor

Right now, the whole Thalmor situation. Overall I'm really impressed by how Bethesda advanced the story between Oblivion and Skyrim, can't wait to see the conflict escalate and find out what happens when the dust settles.

I've also always been fond of the dwemer.

I'm personally ashamed to love TES as much as i do, with this whole civil war thing... Its just asking for trouble, and no Bethesda has to pick a side, without the convienience of a Numidium to cause a time warp so both events happen.

I love everything Dunmer though. They are so atypical of Elves in other worlds. Their bitter, violent, gritty and have a vicious streak that would make a Drow blush, but without the whole pretty-boy approach of their like-skinned D&D counterparts.

Akavir and Dwemer.

It's all so mysterious and epic.

The Altmer, their culture and history, as well as overarching metaphysical lore on a grand level.

tl;dr: muatra as well

Things like KINMUNE or the Loveletter.

Vivec.

I have regressed to the dawn of man's mystical vocabulary and worship Muatra.