this will probably be unpopular but I'd have to say the most overpowered build I've used in recent memory was a Destruction/Restoration mage.  At one point I had to use console commands to remove the impact perk because it completely breaks the combat system and even then I still found it stupidly overpowered.  And this was without using Conjuration or Illusion for a good chunk of the playthrough.  Paralysis, turn undead and impact give you field control that no other build can even come close to touching, as long as you have enough of a magicka reserve (I put everything into Magicka and had over 800 by the endgame, plus enchantments).  If you then add in Conjuration and/or Illusion then there is literally nothing that should touch you, no need to even use stealth you can just waltz into any area and brush enemies aside with little effort.  The master Destruction spells are so horribly unbalanced that it's almost a crime, undead stand no chance against a master destruction mage and everything else may take 1-2 more casts before they're obliterated.  Without using Conjuration/Illusion I think the Mage style is the most overpowered, if I also include those schools then there's not even a debate in my mind.

The staff is completely OP if used correctly! For example, if you are in a corridor if you draw a line from wall to wall then no enemies are able to cross it to get to you (staggers thema nd knocks them back) allowing you to either use ranged attacks to kill them or, as I love to use it, run the hell away from Deathlords! Blackguard is my favourite armour as well.  The best part of it is you can get the full set at a very low level and the enchantments are amazing.  I always used the Guildmaster armour rather than Nightingale since it is so amazing but I now use Blackguard. 

Absolutely.  I typically run Frostfall, RnD, Realistic Fasttravel (disables FT but adds more carts) and immersion tools like iHud.  Add that to SkyRE and I basically play a Woodelf hunter/alchemist that stays away from a lot of adventure and just tries to survive. The SkyRE Wayfarer tree is so enjoyable with a DiD character.  Crafting and using traps that can be poisoned is incredibly immersive.  You can drop a poisoned spike trap around a corner and then draw an enemy into it with a few arrows, such joy! I get more enjoyment from just playing a game of survival (slice of life) style than questing.  I use mods to turn off automatic quest assignments and to delay automatic quests so with 'Alternate start' I never have to start a single quest in the game unless I choose to. 

I've done a few Legendary DiD while also using survival mods like Frostfall...every single mistake is punishing.  It's interesting how many playstyles and habits completely fall away when a pack of 2-3 wolves will end your game in seconds.  I had never bothered with heavy armour or the block skill until I played my first DiD character but the paranoia and fear made me grab any piece of armour that was even slightly stronger than what I had and alchemy was never as crucial!  And stealth becomes insanely intense when you're wearing light armour that you know won't save your ass if anything is alerted.  I literally inched my way through Bleak Falls, probably took me an hour to get passed the sleeping Draugr!  Play with a lighting mod and dungeons become bloody terrifying. Legendary DiD (And even master) is a fantastic challenge, the biggest problem I find is going back to the vanilla game.  Everything is so easy and boring now!

For an embarrassingly long time I felt anything under Master difficulty was unacceptable.  I've come to a change of heart in the past 6 months and now am of the opinion that whatever difficulty gives the character you're using a challenge is the correct choice; there is no 'catch-all' difficulty that one should adhere to.  If that means cranking the difficulty down to Novice so you can play a goofy character that only uses wooden swords...there's nothing wrong with that!  I briefly had a character that relied on 'Marked for Death' and throwing junk at people with Telekinesis and it was actually somewhat viable on Novice! My change of heart came from my dislike of builds that become stupidly overpowered (typically from crafting) to the point where they 1-shot everything and there's no challenge.  It finally sunk in that all that matters is that there is a tangible challenge that makes the character engaging. I recently played a destruction mage for the first time ever (in Skyrim) and found that I needed to set the difficulty to Legendary just to find a decent challenge because Destruction is ridiculously overpowered.  Conversely I've started a new character that uses poison almost exclusively and I've set it to Adept (default difficulty) because poison damage is affected by the difficulty slider and just isn't viable at the highest settings.