I was going to mention ashen grip and nights silence for beginning sets. But I was beaten to it

Now, if you want to get a tad bit more advanced, you can craft one set for the 2 and 3 piece bonuses, and have a full version of the other for the 5 piece bonus. Something I would suggest is maybe 5 pieces of nights silence and 3 pieces of ashen grip. (keep in mind you will need to craft a weapon in one of the sets for the 8th bonus) that way you get the sneak speed, health, weapon damage, stam and stam regen, and crit all at once.

I played around with this alot and ended up doing the following before I could get Hundings rage (6 traits):

1st dagger=Night's Silence (NS)

2nd dagger=Ashen Grip (AG)

Bow=NS

4 pieces of armor=NS

3 pieces of armor=AG

By doing it this way you always maintain the sneak speed bonus, 1xMax Stam, 1 x Stam Regen, 2% Weapon Crit, 1xmax Health, 1xWeapon Damage on both bars. You also pick up an extra 2% crit on your DW bar.

You could swap AG and NS on all the previous items if the 10% chance Fiery Breath proc bonus from AG is more important to you than sneak speed, but IMO sneak speed is much more useful.

Once I could get Hundings Rage it got a little hairy for me. Hundings is only good with the 5 piece bonus giving a significant amount of Weapon Damage, but then you have to sacrifice the sneak speed because you will be unable to have a 5 piece NS set as well.

My current build is Stam Nightblade Vampire (for sneak and regen), 5 piece Hundings, 3 piece Ashen Grip. I may get rid of vampire at some point because I go back and forth on whether or not I like it on this build, but for now I like it so I have fast sneaking and alot of weapon Damage. Without Vamp, Night's Silence is your only way to boost sneak, but the 5 piece limits your other options

For leveling as a NB?..I was amazed at the set that starts a HOT on you after you kill an opponent...I guess not amazed enough to remember the name of it...but i liked the set.

If you Pvp...nights silence.

My recommendation for a leveling character is first of all, no matter what you choose, put the Training Trait on it. The other traits can be nice to have but are not really needed until VR content. It is more important to get your armor skills up to 40 to 50 as soon as you can.

Also, until you get your armor levels into the 30s and unlock the 5 piece passives, there is no need to go all Medium or at least 5 pieces of Medium. Better would be to go with a 3/2/2 (M/H/L) set up. You can still make a crafted set following this scheme. This way you are leveling all your armors and you can then have the freedom end game to change builds or gear and not have to worry about grinding up an armor skill. Armor skills seem to level the slowest, and for sure if you decide to Magic, you don't want to be unskilled in Light Armor for instance. You don't have to spend any points in the armors that are not your preference if your not going to use them in the near future.