Ever wondered who premiered the "hiding in shadows", "taking down enemies silently from behind"and " making noses to distract enemies", that is so prevalent nowadays in many F/TPS?
While there were a lot of other games that paved the way, the genre defining 3D game series was Thief, developed be Looking Glass Studios. The first two games should be considered together, since the second was developed right after the first one and directly continues the story and it has the pretty much the same gameplay.
If you rather want a video introduction to Thief, Errant Signal has a good one
I will try to convince you that it is worthwhile to play one of the early 3D games which pioneered many mechanics that are still used today.
Is it for everyone? No! You need patience and a pretty good orientation sense in order to not get lost and frustrated. But you will be rewarded with 2 good games with clever level design! I played them last year after just trying them a bit when they originally came out. And I had a lot of fun! The gameplay is still entertaining and the worst mechanic is the melee fight. But you are a thief and shouldn't even get in a fight, so from a certain perspective, that fits to the character you are playing.
So what is great:
As said, the level design is exquisite: very varied and fitting to the themes of the mission. A mission inside a bank plays very different from breaking into a prison or a cultists hideout. And they already use a lot of environmental storytelling in these games. Especially the second game has very atmospheric missions, you can see where Dishonoreds roots come from.
And it is also partially an immersive sim: you have a set of tools and are presented hurdles to your objective. How you solve them is up to you.
Tricking the guards and slipping into a building without anybody seeing you is as fun in this game as it is in any model stealth game. And the AI is also already well developed: you can trick them, if they see you you can hide from them in the shadows and honestly, today's guard AI feels very same to this ones.
They also took an interesting and very fitting approach to difficulty: the levels and enemies are pretty much the same, but your objectives change. The higher the difficulty, the more loot you need to steal to proof you really are a master thief, and also you are not allowed to kill em all or anybody, since that is also unfitting to a master thief.
Story wise you are Garrett, master thief and former prodigy of the enigmatic Keepers, who try to uphold the balance in The City. No other name is every given and it seems to be the centre of human activity. The balance is in danger because two factions, the Hammerites and the Pagans are always at each others throats. The Hammerites are a catholic church combined with an industry factory. They are the power of progress and technology and bring order to the chaos. Which is the element of the Pagans, which they revere. They try to bring humanity back to it's original roots in the ancient forest without any fire or technology to conquer nature. Garrett couldn't care less about that and just want to be able to pay rent at the end of the month. Preferably with other peoples money however.
The weakest part of the first game are the fantastical themed levels, which are interesting in themselves, but not really fitting to the stealth game of the normal human missions. The second game nearly fully commits to being thief, greatly increasing my enjoyment of this game. The fantastical elements become understandable when you learn that the game originally was supposed to be an King Arthur themed game.
Best way to play is Thief gold edition and use the mod manager, which allows you to use high Res texture mods, these make playing the games nowadays a bit easier on the eyes.
Also: there are a ton of fan made maps for Thief 1+2, meaning you get so much more content for free.