Thief: The Dark Project.
It's difficult to decide between Thief 1 and Thief 2, especially when fan missions are considered (and nowadays with the NewDark patch they essentially run on the same version of the engine anyway), but when it comes to just the vanilla campaign, the original game wins out for having far more memorable levels.
Thief 2 has a more consistent quality, while Thief 1 has higher highs and lower lows, but even the lows are memorable at least, while when I think back to Thief 2 some of the levels blend together in my memory.
Why is Thief so good? Simple: great atmosphere, great exploration, and stealth gameplay that actually functions and isn't just a frustrating exercise in hiding behind objects and hoping you're not in someone's line of sight. I used to hate stealth in games before, but Thief made me fall in love with it. And to this day there's no better stealth system out there. Sure, the AI is exploitable and I've become so familiar with the engine's quirks that I can exploit it as hard as I want, but the raw systems are superior to anything we got today. The lighting, the sound propagation, and how both interact with AI awareness of the player. Simply excellent.
The amazing atmosphere and wealth of top-tier fan missions are just a cherry on top.
Thief: The Dark Project (TDP) or *GOLD*
In my opinion, TDP excels at everything it does. It has the best overall design of any FPS game. Period. The maps are clever, the missions are varied, the weapons and items are novel, the art is fantastic, the sound design is stellar, the mythology of the world is familiar but fresh, and the gameplay is top-notch. A quarter of a century later (holy fucking shit!), it still does stealth better than any other game on the market. TDP mixes stealth missions with Tombraider-style adventures and horror to make a cocktail that is inebriating to the senses. If you go into the game with no prior knowledge and don't look anything up, like I did back in 1998, you're in for a trip.
TDP's story is hands down my favorite of any video game. Garrett is the quintessential everyman. He's just trying his best to survive in a harsh and unforgiving world. He steals because that is how he eats. It's his job. Unlike that piece of shit nu-Thief from 2014 that recasts Garrett as some emo faggot who steals because of feelings. The OG Garrett is just trying to get by. He's an anti-hero, in the vein of Clint Eastwood in
A Fistful of Dollars. He doesn't let anyone get between him and his money, but he is willing to sacrifice himself for the greater good (or possibly revenge). His is the hero's journey, beginning as a pathetic street urchin who grows up to rebel against the common order and goes on an adventure that ultimately saves and revitalizes the world. Joseph Campbell would surely be satisfied. And best of all, his character's portrayal is given complexity and nuance by the inimitable Stephen Russell (absolute GOAT).
TDP is from the olden times, before games became cutscenes interrupted by moments of gameplay. The majority of the storytelling is cleverly given in mission briefings and within the missions themselves. You have full control over Garrett during the missions - control is never taken away. There are very few cutscenes in TDP, but what is there is presented between the missions and in a unique animated style that is dream-like and timeless. The only thing that has aged about those cutscenes are the resolution. You could easily use TDP's narrative as a blueprint for a Hollywood film.
THAT cutscene reveal that occurs is absolutely brilliant. It was a gut-punch the first time I saw it, and I felt the game crescendo as I went through the successive missions. When Roger Ebert said video games were not art, it was because he had not played
Thief. It is a treasure of a game that should be in the Library of Congress and the Louvre.
TL;DR - best game ever!