If you see well, except Sasori, the Akatsuki that were defeated hadn't any plot relevance, but the rest of Akatsuki: Pain, Zetsu, Itachi, Shounan, Tobi and Kisame. Those are the most strong and have an important plot relevance. Why? Lets see:
Pain: Akatsuki's leader and also the leader of Amegakure and defeated Hanzou, his enemy during the Civil War all by himself.
Zetsu: He acts like Pain's right arm and the other members treat him like a superior. He's power remains a mistery.
Itachi: The one who exterminated Uchiha clan and also Sasuke's brother. It's unknown what's his purpose.
Shounan: Pain's partner. Unknown headband. Need I say more?
Tobi: Revealed as Uchiha Madara and his intentions are different from Pain. Also the third Uchiha survivor.
Kisame: One of the Seven Swordsmans of Kirigakure. He also knows Suigetsu. It's a mistery why he joined to Akatsuki.
That's all. Read and make your own opinions.