9. Old Man Logan
Old Man Logan comes from one of the darkest timelines in the Marvel Multiverse, and when he was first introduced, he was far from strong, or dangerous.
He refused to pop his claws, and hadn't done so for decades. This was due to him being tricked into killing all of the X-Men by Mysterio when the villains banded together to take over the country.
When the Hulk Gang killed his family, it was time to let the Wolverine out of his cage, and he slaughtered the whole clan - including the Incredible Hulk/Bruce Banner, who was stronger than any time in the past. Old Man Logan was definitely strong, but he had some issues he didn't contend with earlier in life.
His healing factor was significantly hindered by the fact that he was 197-years-old at this time in his life. Despite the infirmity, he was still lethal, and insanely deadly to anyone who crossed him.
While he didn't have the healing factor he possessed in his youth, he did have nearly two centuries of experience in being the best at what he did. While it may not be very nice, he was still pretty damn good at killing people... a lot of people.