For real life? Skeleton key maybe,as it also unlocks metaphorical doors,unlocking one's true potential,at least according to skyrim lore. that or The Ogma Infinium.
In being selfish I would say the Skeleton Key,im in some dire need of self improvement lmao. but if I was to choose the Ogma infinium,that knowledge could be used to change a lot in the world.

In game? Namira's Ring or Mace of Molag Bal. Mace would help with Sustain if it Absorbs,and be a rightous pain in pvp if it Damaged or Absorbed. But Ring could give a healing effect or major buff upon killing and activating a synergy so...
But then also...Cowl of Nocturnal. looks horrid but there is a hide helmet option. and the Cowl gives a massive stealth bonus Plus feather in Oblivion,likely translating into Invisibility or Speed....But then ring of Khajiit would allow for a monster set and give invisibility likely,at a cost.

Real life: Skeleton Key. not for lockpicking,but for unlocking said metaphorical doors,allowing myself to become better as a person,helping me get past my own mental illnesses,fears, and all of that stuff.

In Game: Staff of Sheogorath HAHA no Specification on having dedicated Daedric artifacts or noT! I'll use it to Perma CC eNtire Zergs foR my EntertAinment. Ok serious Answer? Staff of Everscamp,provided the Scamps are hostile to my enemies and are fire based instead of shock and work as summoned pets that respawn shortly after dying. I'll even take the snare effect.