Based on the game by Tinker Bell.

Who is the most powerful member in Akatsuki in the Naruto anime?

Obito could have been the most powerful Akatsuki member if he had both his mangekyou sharingan.
As we saw in the manga, Kakashi was able to hold his own against Kaguya when he received both of Obito's sharingan.
Kakashi isn't an Uchiha so it is safe to assume that if Obito used both his sharingan he could have performed amazing feats with the long range kamui and perfect susanoo. However, since Obito never got the chance to use both his eyes to the full potential the title of the most powerful Akatsuki member must go to Itachi Uchiha. Itachi has the unique characteristic of turning any situation to his advantage exemplifying the character of a shinobi What makes Itachi truly deadly is his susanoo's ability to use The Sword of Totsuka, also known as the Sakegari Longsword (酒刈太刀.
This sword can seal any enemy it pierces for eternity.
One slash is all that would take to defeat the enemy.
This technique works on all beings whether mortal or immortal (success rate 100%).
Not to forget the perilous Amaterasu that can burn anything just by staring at them. Although Pain (Nagato) is also very powerful with the rinnegan and the uncanny almighty push, it is arguable whether the almighty push can break the shield of Itachi's Susanoo. However strong the enemy may be Itachi never loses his cool and always has a trick up his sleeve thereby making him the strongest Akatsuki member there has been.