So I've kept my most overpowered save due to role-play (he'll one day take on Mirrak) while playing other characters. But when I got Dawnguard, I soon found out that Vampires were a serious threat. After fighting off a particularily large and powerful swarm in riften with the help of two Storm Atronachs and Mjoll, I decided that the same can not happen to Whiterun, my home.
So I started Smithing. A lot. Eventually I had two full Ebony armour sets for Mjoll and my wife, Ysolda. Mjoll sported Grimsever and an Ebony dagger with extremely potent Shock damage. With the help of reverse-pickpocketing, Ysolda sports full Ebony armour and weaponry favours a mace and shield, so the Ebony mace blasts fire while the shield just eats magic.
And once I started, I continued. I knew NPC's would not keep the armour on themselves, but they were happy to lug weapons around.
So I collected all the Dragon Bones, Ebony and Malachite ingots and Daedric Hearts I could and set to work. After collecting for several days, I had what I needed. A full real-time hour was spent just smithing an array of weapons suitable for a small army and set off to Whiterun, Mjoll lagging behind. She was carrying it all, after all.
When we got there, we were lucky not to face any vampires. I took the weapons from Mjoll and began to dish them out. Those near the gates (guards, glitched Alik'r guards who never left, Adrianne and her husband, the Wood Elf hunter brothers) were given Daedric weaponry on-par with the Artifacts. Each sporting heavy enchantments. Those of the Winds District were given a mix of Ebony and Glass weaponry, knowing that they still needed protection, but it was unlikely that a Vampire would make it that far. Lastly, the Cloud District. Those guys got a mix of Ebony, Daedric and Glass, with the Jarl getting an Ebony Longsword with Damage Magicka and fire enchantments, and a Daedric longsword of Shock and Frost.
The guards were all given Dragonbone. The Ebony and Glass was spent on the citizens and nobles, while the Palace Guards were given the Daedric. I was worried that they would disappear with the Guards, but the guards are only replaced when they die. So a guard that stays alive would stick around with the sword you give him.
The Alik'r warriors guarded the front gate with two Dragonbone Warhammers.
The Vampires came, and were subsequently killed. Again and again. When the Battle for Whiterun began, the citizens stayed at home, but the newly-equipped guards stood firm. More than that, actually. I saved just before the battle, and decided to watch it again, without getting involved. Every guard rushed the Stormcloaks and destroyed them before they could even get past the stables.
TL:DR - 100 Enchanting, Smithing and Pickpocketing = unstoppable NPC's.