I applaud you if you can run even mid level games with no prep time. Some very good DMs can do it.
I spend about 30 minutes prep for every 2 hours of play time (around 2 hours a week for a 6 hour session on the weekend).
I am converting Age of Worms in 5E from 3E though. Most of it is ensuring timeframes for the 6-8 encounter adventuring day is adhered to, and monster conversions (adding mooks to solo encounters, tweaking HP and AC here or there, and appropriate 'counts as' for NPCs).
Party is currently 11th. The EK has GWM (as did the Barbarian before him). The Cleric has bless. Zero issues.
Age of Worms starts off good and goes downhill later IMHO but it may work better under 5E. I thought the Savage Tide was the best 3.5 AP (including the early Paizo ones), with maybe Rise of the Runelords as second and AoW as 3rd.
I'm looking at converting some 1E Dungeon advertures to 5E and ran arough conversion of Moneky Isle for a 1 player 1 DM with 15 mins prep.
With no prep I have likely had some ideas circulating in my head but I will wing it for the most part and pad out the time with a lot of RP. One of our best 5E sessions involved 0 combat and I set the adventure in a Paizo city (I often default to Sasserine her) and the adventure was an ad hoc yachet race that was supposed to be a short sabotage the boat type event but turned into a sailing regatta with several rounds to pick the winner.
PCs ended up sabotaging the boat, getting competing sailors thrown jail over night, getting opposing sailors drunk, sabotaging ropes and sails and dosing booze with laxatives.
I like minimal prep over no prep but if I am running a prepublished adventure for example most of the time you do not need the worlds best combatants and D&D tends to work better without to much min maxing. If the PCs get to anoying you an always pull a tuckers Kobolds on them or Tomb of Horrors.
If I have had a bit of time to prep and pull out the stops on a dungeon hack for example I can be a real bastard. Pit traps into energy draining undead, glyphs of warding on ladders+strength save, pit trap onto a gelatinous cube or overlapping fields of traps so triggering one will set off a chain of traps.
I pulled a good one in 2nd ed, PC picked a fight with an NPC who was mostly a non combatant, NPC triggers a trap sending the PC down a chute covered in blades. The blades had a paralysis poison on it (save at -4 as well), then you get thrown into an ally taking falling damage which triggers another trap dumping you in a room a spear trap with more paralysis poison on it + with a carrion crawler.
From the Ashes (Dungeon 17) has a pit trap that dumps the PCs 200 feet (20d6 falling damage) into a lake of magma (instant death unless protected vs fire then 20d6 damage a round).
In 5E if PCs use things like 10' poles to tap the ground to find a pit trap or use marbles to find slopes I tend to grant advantage on looking for traps.