I'm surprised to not see the Sniper Elite games recommended here, if we're talking single player. They're definitely a bit formulaic by now, and story/dialogue is not worth paying attention to, but they're slow, methodical, and a lot of fun!
There's a bit of a Hitman vibe to the later ones, approaching objectives almost however you'd like, and they can absolutely get hectic if you find yourself in a compromising position.