I really enjoyed Freeside. It had a cool mix of ways to do good for people (Followers of the Apocalypse) as well as ways to extort them or otherwise fulfill their base desires (Fisto). I think Freeside had the most concentrated area for receiving quests in the whole and I always make it a point to go there and make my rounds.

The Dunwich Building in Fallout 3. It was such a cool location, especially with the whole Krivbeknih tie in. Sadly, not as fleshed out as it might have been.
Vanilla Fallout really misses out on going full on with the horror/supernatural elements sometimes.

@quicksilver500 The metro's were a very spooky place. Like what you said with how you had to sneak around in a dark, crumbling metro system. I enjoyed how you would go from a ghoul infested stations to a Raider infested station with fortifications, bodies, gore, etc... which really added to the immersion and made transitions so well. In conclusion I think Bethesda did an amazing job at the metro system for Fallout 3 and I haven't seen anything like that since.

@momuse88 Freeside was interesting like you said with the good people and the bad people either helping or taking. It was also very interesting with all the neon lights and various locations to visit like the Atomic Wrangler and the Kings place. Though I never really spent to much time in there unless it was to do quests as the place was quite a dump with the muggers coming all the time. Especially when the Strip was right beside it which was a lot cleaner. Though something about Freeside just screamed wasteland and showed just how hard it was to live in the Wasteland even with the NCR and Followers.

New Vegas Stip and Dunwich Building probably

The Highway in Lonesome Road was an amazing part for me playing New Vegas. The Dunwich Building was great, real spooky and creepy.

In fnv on the road right next to El Dorado dry lake where there is a dead brahmin and some chewed up corpses with fire ants around it, don't know why but that's what comes to mind that and Springvale right as you exit vault 101.

In NV I'd say The Sierra Madre worldspace. It's probably the dlc you won't get out of your mind anytime coz of that stupid collar around your neck, but also because of that creepy vibe it had, behind every corner could be some ghost or any other threat, that could end your life in a few hits.

In F3 I enjoyed the Point Lookout Swamp area and especially Calvert Mansion. Besides that the many metro tunnels in Fallout 3 (I always hated them coz there's not much stuff in it imo) are pretty memorable.

The Prydwen in Fallout 4 (even if the interior is pretty lame) is my absolute favourite in that game.

@BrittleAngel yeah I know where the Brahmin and fire ants are in FNV, Interesting choice. Springvale could of been a cool location but really lacked much to do other then when you pass if for the first time or when your doing one of the few quests located there.

@Ehlengast I always loved the Sierra Madre because of its Architecture, NPCs, and backstory. The backstory is very interesting and I will provide a link to the Vera's wiki page if you want to read on it. The architecture of the Sierra Madre was really unique and not of anything seen in past Fallout games (I think) and looked very cool. I mostly liked the tile roofs which matches those of the Romans (I Like Romans). Though from the actual DLC story point of view the characters were very interesting with most of them having flushed actual backstories like those of Christine Royce and Dean Domino. And Also God/Dog a super mutant suffering from a multiple-personality disorder. Generally a very cool and as you said spooky location that I really enjoyed for a number of reasons. Really good choice!

Vera Keyes: http://fallout.wikia.com/wiki/Vera_Keyes

For me it would be Takoma Park, tho there is not much special about the location itself, it's really more about what happened to me the first time that i went there.

There was this group of 3 or 4 super mutants, one of them armed with a missile launcher and in front of them was a baby carriage either on top of or surrounded by cars. As i was exiting the metro, going up the escalator i saw it and heard crying coming from the direction of that baby carriage. I looked there and the mutant with the missile launcher fired on one of the cars which caused a chain reaction, all the cars exploded and the crying stopped, now the crying was most likely just the baby carriage bomb trap but it left a strong impression nontheless.

@Ramirez23 Huh cool little unscripted event. Don't think anything like that has ever happened to me in Fallout but still very cool.

@Ehlengast Point lookout was amazing and really brought a whole new atmosphere to the series. I loved the eerie feeling of being in a world that is so out of your element and filled with a culture that you as the Lone Wanderer know nothing about and have to discover what is happening. Awesome stuff my friend.

The Sierra Madre in its entirety, I am a sucker for oppressive atmosphere. I really enjoyed the theme of obsession throughout the environment, the founder being obsessed with his love and the inhabitants being obsessed with his fortune.

I also thought the use of holograms as a thematic substitute for ghosts was pretty clever, and something I could appreciate. This is probably an unpopular opinion, but I actually liked the added challenge in that DLC and thought the level design was very appropriate for the lore of the area.

Despite the rest of Dead Moneys many issues, the Sierra Madre is a place I will never forget in gaming as a whole.

@Youvegotmaelstrom You are not alone at all. Dead Money is honestly my favorite DLC from New Vegas. Very unpopular opinion, i know, but the atmosphere and everything you mentioned is why i love it so much. It has a feeling to it that no other Fallout expansion has and that is why i appreciate it so much.

and as for its many issues, that's why we have mods

@Youvegotmaelstrom Exactly what I said for why I really enjoyed the Sierra Madre. As you stated the story was also very interesting and I enjoyed it very much. Yes the rest of Dead Money was not so good but the Sierra Madre as a location was amazing for reasons you all ready stated in your answer.

@Grimmex The few mods that are released for Dead Money are very good, especially Dragbody's Dead Money Horror Creature Pack which was looks very good.

The Glow in Fallout 1. All you see is desolate corridors of a once alive facility. You see the ash and blackened bodies of what once used to be living, healthy people. Nothing but death and radiation lingers, and the haunting track "Radiation Storm" playing in the back ground.

A very heavy and dark area, one of my favorites from Fallout 1.

@MrEggs0925 Definitely play the Dead Money Horror Overhaul, I just played it on my recent playthrough and it's awesome. Adds a good amount of new locations and it's overall really fun

Most memorable for me was The Cathedral in Fallout 1 where you meet the master in all his cybernetic oozy glory.

My most memorable spot would have to be the spot where you saw the tv in fallout 3 in the trailer, when i found it i was so amazed, yes its lame but i thought it was so awesome, ive been in love with the fallout series ever since i saw that iconic spot in the trailer.

Mine would be The Quarry Junction in New Vegas. How can you forget that deathclaw infested area?

@DarkPopulous I try to block that location from my memory thank you very much. Have you ever been in that place with increased wasteland spawns active? *shudders with horror*