The worst DM I've ever met... well, booya.
A guy who constantly smoked pot during sessions and whose biggest goal in life was to flood the clandestine weed trade in our city with product from the (relatively) nearby Netherlands because he was pissed of with our state's policy regarding soft drugs.
And his game was as one could expect it was. It was not that he was actively malicious, but his complete and utter thoughtlessness when it came to DMing, condoned powergaming beyond belief and seemingly didn't know anything else but a goldrush campaign. Bonus points for hentai-like content as a reminder when something is serious business. In short: He was so utterly immature at 23 that he is my go-to anecdotal evidence when I am stating that I'm fairly sure that too much weed during your teens stifles the development of your personality (and since he had somewhat of a temper, he's my counterevidence that weed automatically has to have a calming effect on people).
How went our campaign?
Well, not good. One was a AD&D 2e campaign. At first we were 8-9 players, with little to now checks of how the party was set up. The whole session consisted of nothing but fights that took ages because we had to go through 10 people rolling for 20 and more different beings each round. Characters casually died and were equally casually replaced with replacements of the players choice of equal level and equipment (so death was relatively cheap here).
I can't recall much of the campaign (another testament of his skill as a DM), but I know that at some points we had to go to Sigil, prompting the reaction of one of our players what an unpleasant and nasty place that was and he wished we hadn't go to go there. I wondered "how bad (not: How hard) can the city of the planes be?" Well, it turned out that he took the fact that "any faction is represented here" as "every member of any faction do as they please and behave as they're used to", which amounted to a vivid description of a gathering place in Sigil were super-endowed demons were casually raping women to death in their section and bypassers didn't seem to care whatsoever. Phat lewtz was also handed out casually, including the retarded Deck of Many Cards where some guy drew the imprisonment card, causing a bunch of Mercy Killers to drag him away in chains, sending us another pointless sidequest to get him back. The DMs reaction? "You're so awesome, with the stuff you get into, I don't even need a plot for the game".
Nevertheless I sticked around (stupid idea, I admit), with the number of the players dwindling (among them also some of his pals, which should have been a huge warning sign for me). We went down to a more manageable size of 4-5 regulars, but the quality of the game didn't improve. The DM constantly needled the most good-natured member of the group (not that the others weren't, he was just extremely laid-back), who coincidentally also was the only one with a girlfriend at that time.
He also was having additional sessions of the side with two of the players, and tailored the challenges to the party to the loot he handed out to them during these sessions, with increasingly ridiculous buffs coming in for them. After he buffed one of them between regular sessions by several levels (I think he went up from 9 to 14), made him a Chosen of some goddess of Magic and handed him out artifact-quality loot and the other guy was pampered similarly, pretty much reducing the other players to spectators to their awesomeness, I quit (though I would have quit as well if I had enjoyed the same treatment because a playstyle that went out of hand to such an extent wasn't exactly what I had signed up for).
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The very same guy also took the cake when it came to DMing a Vampire LARP game. He was co-DM to two other guys, and we were playing as a Camarilla community. I joined as a neonate (I was pretty new at both LARPing and Vampire back then and had little knowledge of the setting, so there was little for me to do back then, and I was willing to stay on the sidelines and gradually get into the game later). I got to play a whopping two sessions.
One reason for this was that we rarely gathered for playing (only once per month) due to the number of players (~15 + 3 DMs). The other was that he caused the completely tanking of the campaign at the second meeting I joined.
To elaborate: While the other two DMs created reasonably powerful (and passive) DM(N)PCs to partake in the setting (two mid-generation Primogens, which made sense), he went all-out and came up with a two thousand year-old Brujah (but he had been in torpor for a thousand years, so it was okay). He also played the role of the Sheriff, since he deemed one of the players, another Brujah, who was interested in the job, as "too irresponsible" if I recall correctly.
The evening was entertaining (it was about a demon and stuff, I only got a fraction of the references because I joined quite late), until the demon temporarily possessed one of our melee heavy players. The player (in character as a possessed being, and empowered by said possession) got into a fight with the sheriff and finished him off permanently in one round, thereby rendering all the work he put into his precisious Marty Stu to dust in a few seconds. This apparently throw him off the rails, and the evening went downhill from that, quite similar to some Star Trek TOS episode, with an immature brat (don't know which race) with superpowers and a childlike mind having pretended to be the demon the whole time for shits and giggles. Judging from the reaction of the other two DMs, this had not been planned to play out like this. He later made a feeble attempt to apologize, mentioning how he pissed off that his DMPC had died, but if someone can't cope with the loss of a (ready made, not actually leveled), ill written wish-fulfillment character and actively ruins the fun for 17 other people because of it, I guess only the most kind would let that slide.
Needless to say, the campaign was discontinued after that.