I'd say orochimaru and pain top 2.
oro was immortal (but a kind of weak version of it) had several strong followers, was basicly a dictator like sadam with thousands of prisoners hundreds of hideouts and turned sasori's spy againest him and was planing on taking sasori out using kabuto as bait. also hes a snake-monster, immune to poisons, made the 2nd hokage's forbidden jutsu even more powerful, killed (kind of) 2 kages, almost brought down the leaf village, was almost impossible to hurt since he took a full power tsunade super punch to the face and only had his mask damaged, and in the anime when he was pissed he kicked down a tree because his arms hurt from rotting. Dude was pure evil and in most any other manga would have been the main villian, and he might still be at the end of the war by taking over kabuto's mind.
Pain could control 6 corpses at once with only his chakra, had a legendary dojutsu, killed trained ninjas when he was an untrained little boy, mastery over any jutsu hes taught as well as the 7 paths of pain which include controling gravity, taking peoples souls, summoning several massive beasts, absorbs any ninjutsu you can throw at him, an android with a small arsonal of missles and lasers and blades in him, and control over life and death itself! oh and he has absolute perfect teamwork with the pathes since they share eye-sight, can make moons (with enough chakra) and wipe out an entire village with one jutsu to teach them a lesson, even his comparedly weak partner almost killed madara the leader of the akatsuki.
After that kisame (loses some points for listening to itachi so well, best friends and all but not very "bad-ass") kazuka, tears hearts out (and someone said he hadn't mastered wind, wrong he lost his earth heart to kakashi at the begining, he used wind the rest of the fight that and fire mask were the last 2 destoryed) then sasori since him and hidan and kazuka were all basicly immortal in different ways. Itachi is powerful and intimadating but he was a good guy in disguise which is awesome but i wouldn't put him as my top "badasses" in this thread atleast.
