1) Kisame: His ability to take chakra away from people via Samehada is really overlooked and overpowered
2) Kakuzu: I think possession of five (including his own) chakra natures along with the corresponding A-Rank Jutsu speaks for itself
3) Sasori: Strongest Puppeteer in existence and has the Iron Sand in his possession. If the secret about his core is unknown to the enemy, he can be truly devastating. Still suspect able to every puppeteers weakness even if he's a puppet himself, as those with extremely high speed (Lee, Gai, Raikage, Naruto, etc.) can blindside him as a puppeteer's Taijutsu is usually sub-par, and this applies to Sasori as well.
4) Deidara: Easily one of the hardest to beat in Akatsuki (C4, C3 and C2 are just overwhelming) if you don't have the Raiton affinity. Took down Gaara by himself. Sasuke made him look bad (honestly, just due to Plot no Jutsu), but a user without knowledge of his C4 beforehand or a dojutsu is pretty much guaranteed a death. His intelligence is actually quite impressive given his somewhat cocky attitude. Even a barrage of his weakest clay, C1 can make it tough on skilled opponents like Gaara, Sasuke and Kakashi. He is of the mid-long range type and his weakness in close combat is ultimately his downfall as very skilled Taijutsu users can take him down mid-difficulty, and Lightning based Jutsu can easily take him down as well. Admitted he was weaker than Sasori, so this has some significance as well.
