Maximum damage in a single hit

Well, as for sneaking up and attacking from behind: you can try using a one-handed weapon(a mace is preferrable, since they have the highest base damage) in conjuction with some of the Sneaking skill Perks(the Dark Brotherhood armour set provides extra sneak attack damage with one-handed weapons as well). Additionally, you can get yourself a high-level warhammer and power attack enemies from behind(this is also a better option if they survive the first hit).

You can also acquire Mehrune's Razor in that questline(initiated in the museum in Dawnstar), which has a chance, albeit a small one, to instantly kill your enemy when hit.

Sneak, enchantment, perks, poison, weapon type, power attack & opponents armor determine the damage of a hit. The strongest damage without cheating or exploiting glitches is to hold dual daggers(daedric or dragonbone with dawnguard), dual power attack while undetected wearing the dark brotherhood gauntlets with assassin's blade perk with poison on it with a preferable enchantment.

Highest damage will be berserk double dagger sneak power attack with dagger boosting perks. The sneak attack will apply to both daggers and will have a 15x damage modifier.

However, all this is meaningless in vanilla. You can abuse the fortify restoration bug with smithing boost gear, and make weapons with thousands of damage, killing everything in one hit.

Expanded: If the goal is short fights, the best way is to focus on Smithing and hit 100 ASAP, then focus on Enchanting so you can make Fortify Smithing gear, then Alchemy to get the Fortify Blacksmith potions. Complete the excavation quest to get enchanting boosting gear in Dragonborn DLC, and also get the Black Book for the enchant and alchemy boost.

This lets you create weapons with damage way above your leveled enemies, and additionally armor that basically makes you unkillable. You don't even need perks at that point.

HIGHEST DAMAGE IN A SINGLE HIT LEGIT: (can only be used 2 times max)

2 Deadric daggers fully upgraded with the best smithing gear and potions, enchanted with fire and lightning/frost using the best enchanting potions.

All armor fortify one handed if possible and the bracers must be the dark brotherhood ones

get the sneak perk that dagger do 15x more damage if it is a stealth attack

then go to blackreach do the crimson nirnroot thing and get that abillity to sometimes get 2 potions or poisons.

Do the dark brotherhood quest line untill the part where you are the gourmet, dont use the ingredient then save it and make a damage health poison, make sure you have made 2 using the crimson nirnroot abillity, apply the poison on both daggers then sneak upp to someone and double hit.

What about maxing out heavy armor and alchemy while playing a khajiit. Enchant your Daedric gauntlets with fortify unarmed strike. The Khajitt racial trait turns unarmed strike to a 15 base, heavy armor adds your armor base to your damage. One glitch allows you to poison your fists... You can use what ever other bonuses you want, but this was very useful to me when fighting miraak.