I was thinking of doing a new build. Now typically, we think of three stereotypes: Melee-based warrior, ranged-based thief, and magic-based mage. Of course there's the occasional hybrid, like thieves who use melee and ranged, or spellswords who use magic and melee, etc etc.

I was thinking of doing an assassin build, but I don't want to use ranged weapons. I want to use daggers, preferably dual-wielding. I want to sneak-in, take out targets with melee, get out.

Would Dark Brotherhood or Thieve's Guild be my best bet? What equipment? One-handed, Light Armor, and Sneak for sure, what other perks?

I'm also up for a "Thane Krios" build. Ambush, one-hit kill everyone , get out.

If you really want to be an effective close-range assassin, I would also suggest going with Illusion and Alteration.

The Dark Brotherhood would be the best guild for you to join, but if you also want to join the Thieves Guild also, be my guest.

Out of that man good luck with your character

Do this. Thane is my favorite ME character. Well, except Mordin, because he's just awesome all around.