The weakest Akatsuki member is Hidan.
IMO, I like him, because he stupid and hilarious. Yeah, he's immortal, but he hasn't shown any particular skills and intelligence (swears excessively). I haven't seen him do any kind of ninjitsu or genjutsu. The only things he's good at is blabbering, swinging a scythe, and getting himself decapitated.
Zetsu is not considered a fighting type ninja, but he's an ideal supplemental ninja. White Zetsu is a great sensory type ninja, since he's able to distinct peoples' chakra easily and know their location. For example, he was able to detect Naruto and Bee when they left the training grounds (the spot they were training in was designed so no ordinary sensory ninja was able to detect it). White Zetsu also has the power to create 100,000 clones and leech off peoples' chakra. And with the chakra he stole, White Zetsu can perfectly transform into ninjas he stole it from; he's able to copy the same appearance, voice, and chakra signature of each individual. This makes him an ideal infiltrator and assassin.
On the other hand, Black Zetsu is considered to "be the land". The reason why this is true is because he has the ability to transport himself to any location in the world. Even though he is not able to transport as fast as Madara, it's still a luxury being able to teleport where ever you wanted to. Black and white together make up Zetsu, an ideal supplementary ninja.
Deidara, on the other hand, is an overall better ninja. He's smarter than Hidan, since he possesses a wide variety of attacks and strategies to take down an opponent. He has the ability to manipulate difference kinds of explosives, from homing torpedoes, mines, and even cell-sized bombs that can eradicate your body from the inside out. Since he has the ability to fly, Deidara can do damage from close to far range, unlike Hidan who can only do close range taijutsu. Under dire circumstances, Deidara is also able to escape death and turn it against his opponent. Deidara's fighting prowess can be seen when he fights Gaara and Sasuke. Versatility is what makes a ninja strong.
All in all, Zetsu and Deidara are better ninja than Hidan. Hidan lacks the ninjutsu, intelligence, and versatility Zetsu and Deidara possess. Even though Hidan killed many ninja, ie Asuma, the reason why they were defeated by this guy is because they didn't know his ability. Once you know about Hidan's ability, you can learn how to beat him by attacking him long range, outsmarting him, and trapping him. If you knew about Zetsu and Deidara's abilities, it would still be difficult to deal with them.
The things that make Hidan a ninja is his immortality, religion, and the rockpit he's stuck in.