Vis Maior Guild from Bonechewer Realm ( US) is the 8th Worldwide in defeating C’thun. Vis Maior is one of the known top guilds to have defeated C’Thun in United States.  The loot reported is:

Vanquished Tentacle of C’Thun
Binds when pick up
Unique
Trinket
Requires lvl 60

Use: Summons a Vanquished Tentacle to your aid for 30 sec.

Mark of C’Thun
Binds on Pick up
Neck
+24 Stamina
Requires lvl 60

Equip: Increased Defense +10
Equip: Increases your chance to dodge an attack by 1%
Equip: Improves your chance to hit by 1%

That’s about one of the best Warrior Necklaces out there.

Retribution Guild from Arthas US Realm also defeated C’Thun and claim to be the Worldwide First Ouro kill.  Death and Taxes Guild (Shattered Hand) defeated C’Thun and Ouro too. Check out a Predestined Guild (Mannoroth)‘s screenshot showing a real-time summoned Vanquished Tentacle below.

Below is a list of the top 20 Guilds that have defeated C’Thun—gathered by Mirokawa (Skullcrusher realm)