What type of armor should I use as a thief?

Solution 1:

It is dependant on your build.

I am playing a Khajit thief and I use light armour. Minor spoiler below:

The thief guild provides you with some nice light armour.

The benefits of light armour are:

• You are less detectable in lower levels with light armour (the perk that light armour weights nothing is at skill level 50, in heavy armour, it is 70). As a thief you don't tend to get hit often, so leveling your armour takes longer than you would expect.
• Windwalker is great for a thief, your stamina regenerates 50% faster if you are wearing all light armour. You need to be agile so stamina is pretty important for you running away, positioning etc.
• Light armour still gives a decent armour rating, so if needed you can go toe to toe without taking a huge beating.
• Fits roleplaying.

The benefits of heavy armour are:

• Higher armour rating
• Tower of strength stops you getting knocked back if you get caught off guard. (You are a bad thief if you get caught off guard ;) )
• You can still sneak effectively once you hit level 70 and get the conditioning perk.

Using robes is also a very viable option, depending on your character set up. If you decide to go this route, I'd recommend going with illusion + alteration.

The benefits are:

• Decent armour rating with alteration stone flesh skins, combined with mage armour. (Triple the effects)
• Robes give +x% magica regen and -x% spell cost on x tree.

This means you can benefit more from the magic attributes below:

• Decent magic resistance from the alteration skill tree (up to 30% magic resistance)
• Alteration grants you the detect life spell. This is awesome as a thief, totally reduces the chances you have of getting caught off guard.
• Illusion skill tree (at level 50) lets you cast spells silently. This is perfect for a stealth character.
• Illusion grants you the invisibility spell, perfect for a thief.
• Illusion gives you crowd control for when the brown stuff hits the fan. You can make a single target (or groups at higher levels) stop fighting, fight each other, or run away.

Solution 2:

Let’s just end the Skyrim assassin build armour debate once and for all, I won’t give monetary values as it is recommended that you keep all the armour pieces because getting extra’s if you lose any is EXTREMELY difficult especially with the advanced sets (Nightingale; Guild Master’s and Ancient Shrouded).

Okay, so first we’ll look at Shrouded Armour vs. Thieves Guild Armour (the easiest to acquire):

                  Armour  Weight   Bonus

Shrouded Cowl         13       2   +20% bow damage
         Armour       29       7   +50% poison resistance
         Gloves        8       2   +100% backstab damage
         Boots         8       2   Muffled movement

Thieves Guild Hood    15       1½  +10% better prices
              Armour  33       7   +20 carry weight
              Gloves  10       1   Lockpicking is 15% easier
              Boots   10       1½  Pickpocket success is 15% better

All in all, Guild Armour is better for thief-oriented classes who will use Lockpicking and Pickpocketing more, whereas Shrouded Armour is much better for assassin classes. Shrouded Gloves remain valuable even when you’re a high level (if you miss the Ancient variety).

Now a look at the Shrouded Armour’s alternate counterparts, the Dark Brotherhood offers Ancient Shrouded Armour (only if you kill Gaius Maro in a major city and use Olva’s Token when talking to Olva, then retrieve it from a secret room in a ruin) which has higher armour values and much higher enchantments. It also offers Shrouded Robes and Worn Shrouded Armour. The Robes are better for mage-assassins and the Worn variety is unenchanted, meaning you can get whatever enchantments you want on it. There is also the Jesters clothes and Cicero’s clothes. These are excellent if you are an assassin who uses alteration spells instead of armour or are a stealthy mage looking for stealthy gear.

                  Armour  Weight   Bonus

Shrouded Hood          0        ½  25% better sneaking
         Robes         0        ½  -15% Destruction spells cost
         Hand Wraps    0        ½  ×2 backstab damage
         Shoes         0        ½  Muffled movement  

Ancient Shrouded Cowl 19       1   +35% bow damage
               Armour 41       5   +100% poison resistance
               Gloves 15       1   ×2 backstab damage
                Boots 15        ½  Muffled movement

Jester's Hat           0        ½  +30% to sneak
         Clothes       0       1   Prices 12% better, +12% one-handed damage  
         Boots         0        ½  Wearer is muffled and moves silently
         Gloves        0        ½  ×2 one-handed damage in sneak attacks 

Cicero's Hat           0        ½  +35% to sneak
         Clothes       0       1   Prices 20% better, +20% one-handed damage
         Boots         0        ½  Wearer is muffled and moves silently
         Gloves        0        ½  ×2 one-handed damage in sneak attacks

So now we come to the most debated armour’s in Skyrim, the Nightingale Armour. I’ve met people who only use this because it looks good, I’ve also met people who hate it because they got it at a low level or don’t use the skills it gives you. People thought they’d get an upgraded Thieves Guild Armour, but Nightingale Armour is designed for Nightblades, stealthy mage-assassins who can hold their own in a swordfight. It is a mixture of combat, thief and mage armours. Well, here are it’s values at the highest level, 32 or above when receiving the quest ‘Trinity Restored’, not when you get the armour! The armour stats don’t change but the enchantments get stronger.

                  Armour  Weight   Bonus

Nightingale Hood      15       2   -17% to Illusions cost  
            Armour    34      12   +40 stamina, +50% Frost Resistance
            Gloves    10       2   +25% to Lockpicking, +25% one-handed damage
            Boots     10       2   Wearer is muffled and moves silently

Note that this is the highest level muffle achievable without crafting your own boots. The value between level 32+ Nightingale Boots and level 19+ (second highest) is over 5000 septims, meaning that at the highest level the Boots are the most valuable thing a Nightingale wears. This also makes the Nightingale Boots one of the best boots for ANY stealthy character.

And finally we reach the only Thieves Guild Armour that looks different to the standard and has better enchantments. The Guild Master’s Armour, NPC’s that wear are senior Guild members like Vex (Master Picklock), Delvin (Master Sneaker), Brynjolf (Second-in-Command) and Mercer Frey, the Master of the Thieves Guild. This armour can only be received after completing the main Thieves Guild Quest Chain and all the Quests that restore the Thieves Guilds influence in the Major Holds, you get these quests of Delvin and Vex after completing enough odd jobs in a particular Major Hold. Linwe’s Armour is encountered during one of these quests, ‘Summerset Shadows’.

                  Armour  Weight   Bonus

Guild Master's Hood   16       3   Prices are 20% better
               Armour 38      10   +50 to carrying capacity
               Boots  11       2   Pickpocket success is 35% better
               Gloves 11       2   Lockpicking is 35% easier

Linwe's Hood          16       2   +15% bow damage  
        Armour        31       6   +15 to stamina
        Gloves        11       2   +15% one-Handed damage
        Boots         11       2   +15% to sneaking

Pick whatever armour suits your character the best, or make your own. This is just to let you see the stats of all the different Thief/Assassin armours before you spend an hour getting some stuff you won't use.