I want a good and long quest line for my new character I am about to create.
Any suggestions?

Depends on what kind of character you're making. My favorite kind of character is a thief, so my favorite questline is thieve's guild. For a warrior I'd say Companions questline then Dawnguard. Mage, do a bit of the main quest to get your level up then go for the College of Winterhold questline. Like I said depends on the character you're making.

Im fond of 'Lost to the Ages', the sidequest from Dawnguard.

Its a nice long quest with interesting puzzles that you can figure out using clues left in journals and on scraps of paper.
What I really like about it is that if you havent discovered the quest locations yet, there is no magic arrow pointing you the way.
You figure out where to go using that journal and when you get close to the location, the arrow appears.

It also has a really good story and interesting rewards.

I like the Mage College plotline, myself.

My favorite Questline in the game is the Dark Brotherhood. I'd recommend you check it out, if you haven't already.

I hear there are some excellent quests in Dawnguard, but as a PS3 player I haven't yet had the chance to try them.