But, if I have to pick one that just piqued my interest from the very start, it is the Shield Guardian from 3E. The very first time I read about them, I immediately developed an adventure idea involving a tower, a long-dead wizard, and actual, in-game consequences for Alignment choices. And what's more, I successfully ran it, and it worked exactly how I wanted it to.
From the SRD Wiki:
Created by spellcasters to be bodyguards, shield guardians are constructs that protect their masters with spells and stamina. When it is fashioned, a shield guardian is keyed to a particular magical amulet. Henceforth, it regards the wearer of that amulet to be its master, protecting and following that individual everywhere (unless specifically commanded not to do so).
A shield guardian obeys its master’s verbal commands to the best of its ability, although it is not good for much beyond combat and possibly simple manual labor. It can also be keyed to perform specific tasks at specific times or when certain conditions are met. The wearer of the amulet can call the shield guardian from any distance, and it will come as long as it is on the same plane.