Okay, first of all, the "Best Armor" for an Assassin is a fairly minor issue, if you play like an Assassin. By this I mean avoiding direct Conflict/rontation to steal, and kill undetected. If you're good enough at this, eventually you just don't get hit, so Armor is basically Enchantment Carrier with a tree for certain useful Perks. You're right about Light Armor, Wind Walker is always nice, and the best pre-enchanted pieces for this style are almost exclusively Light Armor. I contend that they might as well be Clothes, except when it all goes wrong, and you're forced into Fight/Flight.
Dragonscale has the highest Armor Rating, that's it. If you're Smithing, you can get it to the cap, just like heavy, and that's fine, but you end up investing an offal lot in something that you only ever need if 1) they detect you, 2) they close enough to be a threat, 3) you don't kill them first, or 4) Escape to sneak back around, and 5) can land enough blows, or surround you to beat you down.
This makes Dragoonscale ideal for a Fighter who has a lot of Assassin Skills, but prefers to wade in, and tank. If you're not going to tank, then you don't need all that much Armor Rating. A minimum will do, I've gotten away with nothing but Health once I got to a high enough level (In Clothes, with a Greatsword so I couldn't use the Shrouded handwraps, nor Assassin's Blade.)
Otherwise, it's a heck of a lot simpler to go get the Shrouded gear, then Nightingale, and go around to grab the scattered bits, and pieces to dial it in. Pay attention to the Effect they give you (Some of which can't be Enchanted) and all but ignore the Armor Rating. Late game, you might want to get the Nightingale/Ancient Shrouded for that extra little bit of protection, but for the most part, this is the least important thing to an Assassin.
Weight means more, because Silent Roll is still a Sprint, and the more your Equipped items weigh, the more Stamina it takes. So, if you go with Elven, then you can recover from a roll faster (With Windwalker) and roll more often while still having enough Stamina to Power Attack from behind. This goes for Eagle Eye/Steady Hand, Critical Charge, and everything else you use Stamina for. If you use it all up Sprinting, you don't have enough left for a Power Attack (or Bash, long term Zoom, whatever) or have to recover longer/more often.
If you keep your equipment light enough (With my Arcane Assassins, 0.) then, you don't have to invest in Stamina, at all, ever. Non magical, so you don't need Magicka either, so you can put it all in Health. My picks for that (Unarmored) are the Shrouded hood, Jester's Gloves/Boots, and whatever you want on your body. (Either the Mourners' Dress, or Ulfric's Clothes, depending on the Gender.)
As an Archer, I suggest Linwe's Hood, Gauntlets of the Old Gods, Nightingale Boots, Guildmaster's Armor (or whatever, I like that jacket.) and whatever you can whip up for a bow. I kind of play caddy between the Drainspell/Blackbow (For Mages) Nightingale Bow/Finerial's End (Massed Bandits, and the like because of Slow, any Cold Enchanted one will do) and whatever I find lying around, because I rarely Enchant myself as an Archer (Alchemy works better.)
For backstabbing/light skirmishing, I prefer a Sword/Dagger combo, because I can get the Dual Flurry fast striking, Savagery damage bonus (on top of Savage Strike) decent Reach (The main shortcoming of Daggers) and Assassin's Blade multiplier. So, basically whatever the situation, I have the right Blade at hand. The Blade of Woe is sword slow, but otherwise does the most damage, can heal you on strike, and also works with Assassin's Blade. Not too hard to get, either, I can pick one up within a couple hours (IRT) from leaving Helgen with Fast Travel.
The best Swords are the Nightingale Blade, Windshear, the Drainheart Sword, and anything you can make up. Eventually, you can enchant Windshear with Absorb Health/Stamina, and be set. The Scimitar also pretty much eliminates the need for Armor, as long as you don't get surrounded by more than you can stunlock (I've gotten up to 5 with Elemental/Flurry) or stand still in front of too many Archers. Absorb Health means you can slug away unarmored as long as you can strike faster enough to outheal the damage they deal, and Absorb Stamina keeps you running around the battlefield, Critical Charging, Silent Rolling, and Standing Power Attacking through your Backstabs.
The Drainheart, because it's the lightest sword in the game, so everything I said about Armor Weight, only for Power Attacks, instead of Sprinting. So, for a Critical Charge, the Sprint portion drains you a minimum for the lightest possible Armor (if any) then the Power Attack burns a minimum because the DHS weighs less than most daggers, then you get 15 points of Stamina back on strike to keep Sprinting, or follow up with another Power Attack... If you Dual Power Attack with the NGB, and BoW, you get enough stamina back to keep spinning, and probably more Health back than they can take without overwhelming numbers, AFTER you take the biggest danger with a Backstab!
In short, Armor Rating is for stand up fighters, Stealth is your primary defense, range your secondary, evasion your tertiary, and after all that fails, you have to worry about damage Reduction/Health. When all you have is Armor/Health, the rating is a lot more critical.
