Golurk
One of the best rock setters in the tier, if not the best due to its infamous No Guard Dynamicpunch. Even without Dynamicpunch or Stealth Rock it can be a very difficult to defeat, as Iron Fist is a fantastic ability and lets it hit like a beast. The only other choices for me were Amoongus, Ludicolo and Mesprit. Mesprit was only around for a short while and didn't really leave too much of an impact after it left. Amoongus was a really cool sleep inducer with great defenses and Regenerator, but was destroyed by anything with Taunt or Vital Spirit and quite honestly didn't leave too much of an impact, as all it really did was sit on your team and put something to sleep. Ludicolo is a really good Pokemon, with good defenses and fantastic offenses in the rain, but just can't stay alive for very long, and it also didn't really affect the metagame much.
Golurk just left more of an impression on the meta as a whole, while the other three were more self contained threats.