I personally enjoy Invaders, Coal Chamber, and the Threnal Ruins Series the most.
I like the Atraxia's Haven quests, as well as Relic of a Sovereign Past (we need more duregar to kill I think)
They all seem to have some challenges and are fun to play both solo and in a group.
Yes protecting Coyle is a pain, but its a challenge, so it makes things more enjoyable.
Wiz King, Shadow Crypt, Deleras, and Gianthold are great also, but after repeating so many times, burnout sets in.
Like I said, its not about xp/min or loot, just the enjoyment that you get out of the quest and why.
Thanks!