Hello all.
I'm thinking about picking up Oblivion again so what are your guys favourite quest mods?

Perhaps one of the mods listed there is to your liking: http://www.listoid.com/list/145 (it's old stuff, I haven't convinced myself that the download links for the individual mods still work).
Another example is Reclaiming Sancre Tor, or, if you understand German, Lord Kain's Adash mods (I don't know if an English version exists).
And of course Nehrim, the huge total conversion.

Have fun!

Thanks for all your suggestions, some of the mods from the list look like they could seamlessly fit in the TES lore. I'm interested to try Reclaiming Sancre Tor, Knights of the Nine Revelation, Integration: The Stranded Light, Servant of the Dawn and Knights of the White Stallion. Adash is voiced in German but has English subtitles and it looks great, my only concern is how well it would integrate with the lore.
I would play anything from the creators of Enderal.
Again thank you! The Unspeakable One graces my first topic, I'm honoured. Lost Spires sounds interesting, but like Adash not sure how it would merge with the original backstory.
Ps. Wasn't sure if I should make an H.P. Lovecraft or a Pratchett/Gaiman reference.

That's Nehrim - but totally unrelated to the Tamriel lore. Have a look at sureai.net.

There is also a mod called Tamrielic Lore. Never played it myself. And a thread you may find interesting: https://forums.nexusmods.com/index.php?/topic/998311-most-lore-friendly-mods/.

As an aside: Please make sure to check the readme files with respect to requirements and dependancies / incompatibilities / load order.
Not only the technical ones: Lost Spires, for example, is pretty tough for a low-level character.