Thief is a stealth game about a guy who needs to make rent so he breaks into the most dangerous places possible in order to steal ancient relics, then some weird stuff happens.
The game is surprisingly easy to get into for a game this old (after remapping the controls, of course), the mechanics are very intuitive and there isn't a lot of structure on how to complete objectives, you just go at your own pace with whatever tools you want. Obviously some strategies are better than others, but the gameplay allows for experimentation.
This title has a unique approach to difficulty as well, raising the difficulty adds a few more enemies, but also new objectives to complete which make a significant difference on the longevity of the experience and makes it worth it to explore the wonderful levels Thief has to offer. You really feel like you are a thief sometimes, searching everywhere you can for valuables.
Very easy game to recommend, both for its qualities and historical significance for the medium, it's like a dollar during sales too.