Mine is probably the Liberty City Minute achievement for Grand Theft Auto IV. I think technically you have 30 hours to beat the main storyline but I managed to power through and beat the entire game in one sitting.

Achievements were only added in after an update a couple years ago (when they removed the dated Games For Windows Live DRM that made the game pretty much impossible to play without modding) so it's technically not the rarest for the game. The achievements for getting the highscore in QUB3D and flying underneath the bridges are even more rare but I'd argue they're easier to get.

What Steam achievement are you most proud of?

Is there one that took a lot of time to get? Did you have a good time with friends? Was it an achievement you struggled to get on other platforms when you were younger?

Regardless I'd be interested to hear about it.