Let's go through the individual pieces, shall we?
Head: Nightingale Hood wins on looks, but the Illusion bonus is only useful for Nightblades, or other builds that actually use it. (TBPH, 20% off Illusion isn't all that great if you have any Magicka Regeneration, because you can hold your Strike until you recover for the next Casting.) 2nd place probably goes to the Shrouded Hood, the 15% to Sneak is nice at lower level, and it's arguably the best looking hood in the game. The Ancient/Maskless/Shrouded Cowl is nice for Archers, but leaves a gap around the neck which I don't like. Linwe's Hood doesn't clash with either set, and is tied with the ASC for Enchantment.
Body: What do you want here? Again, the Nightingale Cape is kinda neat looking, the high collar matches up to the Shrouded Cowls better. Stamina is nice, Frost Resistance okay, I guess. Not a real high priority for someone who's main defence is never being seen. Poison Resistance is again, Okay, you will probably run into a lot of Dart Traps, but not what I would call lifesaving. The TG/GM's/Linwe's Armor is nice for Carry Capacity, and can look pretty good mixed, and matched with some of the other sets...
Gloves/Gauntlets: Shrouded, Hand Down. Double Backstab, or Fortify 1h that doesn't even work with Daggers!!? Tough call. The Jester's gloves match well with Robes, and other clothes, not to mention the boots.
Boots: Pick one. They all have Muffle, some have better Armor, depending on the Level you get the Nightingale set at, the Shrouded ones could be higher, but again, Armor is for when they see you before they die, your last ditch defence when everything else goes wrong. I pick the ones which match my outfit best, usually the Shrouded ones to go with the gloves.