Deidara. I pretty much liked most Akatsuki, but he was the best one.
Hes the first major real villain that was used in such a way that immediately set the tone for the second part, as stakes were higher and battles would be tougher for the main cast. The Akatsuki were hyped as this organisation consisting of these incredibly powerful ninjas that were feared across the land, and were on par with the toughest of shinobis, such as the Kages or Orochimaru. We got a taste of them when Itachi handled like three Jounin at the same time, but it was just a taste. This guy showed us just how incredibly powerful just one of these basterds were.
Basically, Deidara was the one to give us the first impression of how fights were gonna be like. Not only that, his first major acomplisment was defeating Gaara, one of the most powerful characters in the first part. In that fight, Deidara used both his unusual power and his wits to defeat Gaara, something quite rare in Naruto. Not much later he almost kills like the entirety of team Kakashi and Guy with no hands, and only failed to do so because......Sharingan.
Sharingan is the bane of Deidaras existance. He considers himself an artist and cares for his creations which he uses as weapons, so him being so easily defeated by Itachi was a major blow to his confidence as an artist. He wants to regain that confidence by being able to come up with something that could completely counter it.
Basically, his story is about him overcoming his weaknesses so that he could take pride in his creations. Every bomb he made was a masterpiece to him, thats why he was so careful with it, and wouldnt bring more clay than he needed to a battle. A quite original goal for a villain. Of course, like almost every great idea Kishimoto has ever had, it takes backseat to the Sharingan or whatever bullshit Naruto is spewing at the moment.
Ultimately, all of the Akatsukis potential wa squandered by more cliche developments, and some bizzare ones as well. The Akatsuki ultimately lived up to their hype, but then were traded for villains like Madara or Obito, who really didnt matter anyway as Kaguya was the actual final boss. Deidara, fortunetely, blew himself up before any of this could happen.
Of course, Kishi wasnt satisfied with his suicide being in vain and brought him back only to lose to Sai. Yes, the guy that defeated Gaara and almost defeated Sasuke lost to Sai. The very chapter this happend i dropped Naruto. Kishi just forgot whatever the hell he was doing.