temple

Ummm, which one? The Temple has about 23 quests.

I always really loved http://www.uesp.net/wiki/Morrowind:Meet_Sul-Matuul. That's where the Main Quest gets really into the mythology of the Nerevarine and it really thrilled me the first time I played it.
You quickly go from being a mere Blade representative to actually fulfilling the Prophecies.

And the Urshilaku Burial Caverns are one of my favourite vanilla dungeons.

One of my favorite quests (and my favorite vanilla dungeon, too!) was the mission to confront Dagoth Gares. Scared the heck out of me the first time through, I must admit.

Holamayan,first time there really gave me a purpose of things behind it all.

second meet sul-matuul for pretty much the same reasons.

I enjoy the long quest to be hortator of all the Great Houses I don't know why it just was fun since it made you have to go all around Vvardenfell's main cities.

And the quest to be (forgot the name) of the tribes I really like how the tribes are all the rules and there way of living intrigued me

All those naked Nords who got tricked by witches. :drool:

Seriously, I'm not sure there are just too many awesome ones

I like the off-journal quest that starts in Nerano tomb and ends with the bow of shadows

Always been Disappearance of the Dwarves from Trebonius. It's not that hard but you should keep on your reading to really enjoy it.