Besides Knights of the Nine and Shivering Isles of course.

I guess I'd say the Thieves Den. Oblivion can be a lonely game and this DLC adds a fun little underground 'village' of characters that I quite like.

Mehrunes Razor although I did it only once. The setting, characters and the whole story are really well-done.
Spell Tomes is a nice addition as well.

I never use other DLCs, I (or should I say my characters) don't need several homes / hideouts across Cyrodiil.

I was going to buy one but Other Than Knights of the Nine and Shivering Isles which were both amazing I never did. So I have heard both of those So should I add a poll to the top?