Despite essentially codifying much of the genre, Thief stands head and shoulders over most of the stealth genre even today, in my opinion. Every single mechanic, from the light gem that shows your current visibility to the difficulties giving you more objectives and restrictions rather than arbitrarily cranking up some stats, is so well thought-out and meshes so well with the open, layered level design that it's honestly impressive this isn't a sequel to some much more poorly aged first game.

Some people might take issue with the less stealth-focused levels and while I agree that they're the worst half of the game, I still think there's a lot to love about them. Also, the worldbuilding and atmosphere are extremely good, as is Garrett's character.

That said the Thieves' Guild sucks on anything higher than Normal, honestly just use the level skip cheat code if you want, or play it on the lowest difficulty.