Who has the most accomplishments between C'Thun, Yogg-Saron, Y'Shaarj, Xal'atath, and N'Zoth?
C'Thun is the obvious loser of this competition since he is only responsible for the War of Shifting Sands and the Ahn'Qiraj event in WoW. In his death, he also helped empower Cho'gall since Cho'gall took some of C'Thun's lingering power.
Yogg-Saron is responsible for the corruption of Loken and the other 4 Ulduar Keepers (though they weren't corrupted in the same way Loken was), the death of Tyr, some of the vrykul, mechagnomes, and earthen, along with Archaedas and Ironaya, fleeing to the Eastern Kingdoms area of ancient Kalimdor, and the creation of the Emerald Nightmare. Based on his visions in Ulduar, he also might have something to do with Llane's assassination and the Lich King.
Y'Shaarj's power resulted in the creation of the Well of Eternity since the Titans were forced to pull him off of Azeroth, he is responsible for the creation of the Sha, and his heart empowered some of the True Horde and destroyed a large chunk of the Vale of Eternal Blossoms.
Xal'atath caused the Troll-Aqir War by tricking Zan'do into awakening Kith'ix, and this war would ultimately cause the scattering of the trolls across the planets, which allowed for the Dark Trolls to become Night Elves. She also created the curse on Grim Batol, and she encouraged Natalie Seline to create the Cult of Forgotten Shadows. She is also helping the High Priest PC during the events of Legion.
N'Zoth is responsible for the creation of the Naga, and he greatly expanded the influence of the Nightmare by corrupting Xavius. He is also the one who was the master of Deathwing during the Cataclysm. It is heavily implied that he will escape from his prison by the start of the next expansion, so he is bound to do much more.
I'd rank them like this:
1. Yogg-Saron
2. N'Zoth
3. Y'Shaarj
4. Xal'atath
5. C'Thun
N'Zoth will likely become #1 after the next expansion.