"You will not stop me this time, Jedis. Hah hah, I said it. Jedis. I'll say it again. Jedis..."
Darth Grievous was the imaginary alter ego of General Grievous. Unlike the general, he totally kicked Jedi ass. He never lost a fight. He was known to destroy your planet for no reason whatsoever. He was a killer of over 16,000 Jedi and was fond of referring to them as "Jedis", much to their annoyance.
General Grievous was trained by Count Dooku, which was the reason he lost. Darth Grievous, on the other hand, did not train under the Count. Moreover, Dookie did not even know about his existence. When Count Dooku was training the General, Darth Sidious was concurrently secretly training Darth Grievous in the ultimate Sith classroom. The only Jedi who could top Darth Grievous was the frog dude, though they never met.*Dammit...*