[This is all on my second character, an assassin 1-handed dagger/archer]

I beat the dark brotherhood questline, I'm almost done with the thieves guild now, and my sneak is super high. I can sneak up in front of anything (except for former adventurers apparently) and 1-hit kill it, including giants. What other quest lines are there that are really good? I haven't beaten the main quest line on my main character, but I'm pretty far. I think I'm at the part after you meet *spoiler* paanthrax, or whatever he is called *spoiler*.

The Forsworn quest in Markarth was really good, as are most of the Daedric quests. I definitely like that they are more a part of the world instead of Oblivion's "Find Shrine, come back at right level, then hear a despondent voice tell you what to do" version of them.

As for me, I make one character for each major guild, and then do the main quest with one of them, or make a new character specifically for it.