I would say, among those, for a Nightblade... obviously the Dark Brotherhood questline around the Gold Coast! Do it all, assassinate the big boss... and then... enjoy the most epic sidequest of all, the hunt for the sweetroll killer!

Of the rest here...

Dealing with dominion hatchling-killers in Shadowfen is a decent questline, as is saving the skald king from assassins and insurrection in Eastmarch (some of the quests there make you feel all cloak-and-dagger counter-espionage agent nightbladeish!), and dealing with reachmen and worm cultists (and the ghost giant they let loose) in the Rift is also all right.

Necromancer hunting in Alik'r is neat, fighting first the reachmen and then imperials in Bangkorai is okay... the ending there seems a little out of the blue (where exactly did general tharn get her info? And how come high king emric is such a doofus as to personally run into practically -the most obvious- trap imaginable with no precautions at all?), but... eh.

Coldharbour of course is nifty, first reassembling your scattered forces, then assaulting molag bal in his home... messing up his planemelding plans, and after that finally getting the big showdown and completing the mainstory...

Craglorn... has a decent questline, though some of the side questlines are still groupy. Which makes it somewhat unrewarding if you play through the solo-ized content, but cannot finish the rest due to lack of friends...

An the thieves guild at Hew's bane is very nice, through a bit "deja vue" with the dark brotherhood questline... whichever you do first, the other will feel a bit too much like it in some details.

And while we're talking storyline... other then those mentioned... I definitely enjoyed the Morrowind questline in Vvardenfell most (especially for the plot twist once you think you have it all figured out and fixed, when suddenly some things are not as they appear and the whole matter goes to the dogs... dog... well, you know... Especially since you then may realize you saw the main culprit in the background right at the start, but didn't pay enough attention to realize what was going on - how could you have, after all...), closely followed by the Orsinium plot in Wrothgar (which was was more predictable, but still quite epic!).