One of the all time great DLC expansions for a RPG. I'd say that it easily tops Mass Effect 2: Lair of the Shadow Broker.

Fallout: New Vegas is a very immersive game, especially with Hardcore Mode enabled, where you have to worry about hunger, tiredness, hydration, etc.

Another thing that makes New Vegas so immersive is the literature component crafted by Obsidian's superb writers - they're a league of their own, Bioware's and Bethesda's are very poor in comparison. In a RPG, good writing can contribute greatly to creating a believable, living world for one to lose themselves within. NV is a case where this is very true.

Some exceptions withstanding, FO3's Capital Wasteland is populated by 2D cartoon characters and everything is governed by blatantly black and white morality. The majority of the time there is next no middle-ground in Fallout 3, you're either a total saint or a moustache-twirling villain. Hmmm, should I blow up Megaton or not? HARD CHOICE, EH?

Quake's SP was fucking grand. great level design, awesome sound work and especially immense atmosphere.

To this day I can go back and totally immerse myself in Quake 1, with it's highly atmospheric grungy Gothic setting and design, ambient sound design and industrial soundtrack, creatively made monsters, etc. Do so with a source port such as Darkplaces, that has full dynamic shadow mapping and the experience is deepened even more.

The shriek of a Vore or the roar of a Shambler would always send chills down my spine. Even with the arsenal of a sci-fi Rambo at hand, those monsters were still intimidating opponents. Even the roar of the Orges was always a sign that I should keep my wits about me, because of their devastating grenades.