Gotta say, I never liked Old World Blues. I went into it hearing that it was a great DLC, but I just found myself disappointed with it. After the first playthrough, it's become one of those sections in games that I just want to breeze through as fast as possible.

I love the design though, the vibrant neon colour palette and map itself are awesome, and some of the enemy designs are genuinely quite creepy. But in terms of the quests, the amount of backtracking and many of the 'characters', I'm just not a fan. I can see why many people love it, though!

Just off the top of my head mothership zeta, even when i went into it excited about aliens and being in a spaceship good god was i dissapointed, even as a kid who gave kids so much leeway it was an absolute slog. runner up is definetly Operation Anchorage. How the fuck they took such amazing and brilliant concepts for DLC and just absolutely ruined it is insane. I mean i get its todd howards bethesda but jesus christ man how do you botch it so much. The battlefields of the lead up to the great war should not be so boring

I personally only really dislike going to Zion just because I find it boring af besides daddy Graham.

Hm.. My Fallout experiences are limited only to New Vegas and 4. Worst DLC..
I'd honestly have to say it's a toss up, or both for different reasons between 4's Automatron and Nuka World.

Automatron just felt particularly shallow, increasingly so the more times I played it. I don't know, I suppose I just feel that they could have done more, had more options for robot creations, and the Tesla Rifle was a particular disappointment. Though I suppose that's more in part to my being spoiled by the Tesla Rifle mod from New Vegas prior.

Nuka World feels like a desperate overcompensation for the lack of "evil choices" and, heh, to quote princevegeta2005's opinion on Anchorage, "How the fuck they took such amazing and brilliant concepts for DLC and just absolutely ruined it is insane."
The chance to be the exact opposite of the Minutemen Glorified Errand Boy "General".To actually be a leader of Raiders, but the more you dissect it, the more you realize it's just literally just the same Minutemen crappy-pasta but and Raiders are just swapped Settles, so ultimately.. Yeah. It's increasingly, staggeringly insulting to make you go through the same crap twice.
And yet they slack on this to still have Gravy on a pedestal of untouchable and all in all, there is no real consequence, and par for the course with 4, your choices don't actually matter after all.

Having looked at the two this way, and really seeing it for what it is - Definitely Nuka World for me.

Speaking broadly? Mothership Zeta's probably the worst. It's way out there by the tone of the rest of the series, introduces all sorts of logical gaps that make it hard to explain exactly why the Lone Wanderer doesn't just hand the craft over to, say, the Brotherhood or whatever faction of your choosing and set up their own go at bringing order to the wastes...plus it just isn't fun. At least it gives new energy weapons, I guess.

Of the NV ones...I choose Lonesome Road. Mainly, I can't stand Avellone's mouthpiece characters and, even as I conceptually like Ulysses, how he's handled in-game does enough to turn me off the DLC. That and, as with Zeta, I don't find it particularly fun to play.

Zion bored me a lot tbh, the redeeming part of that expansion was Graham. Badass character, very well made with a very distinct personality.
Storywise, i felt the whole DLC rather empty.

Re-L Mayer wrote:

I really dislike the Mothership Zeta DLC, the only good thing that came out of it, was the mod that was inspired by it.

There's a mod inspired by it? As someone who actually really liked MZ I'm intrigued !

By far, Wasteland Workshop, for real. It should have been in the main game, that's it. 
From what I've seen everyone really dislikes MZ. I understand why, but in my case I kinda enjoyed it DLC actually, it isn't great (not at all) but it isn't that bad IMO, my only problem with that DLC was that the story actually has 0 impact, unlike HH (which is the other hated DLC) which actually gives some insight about part of the lore and introduces an amazing character who in the base game was practically a legend (Joshua Graham). MZ was short, kinda rushed from what I remember (like 5 years since i last played FO3) and had no fully made or at least lacked of some proper memorable story.