If you were to make a rankings based on impact, pioneering, or being larger than life than Johnson probably rates right at the top of heavyweight lists but if you rank by the caliber of wins/losses than Johnson shouldn't be anywhere near number one and I think he has trouble cracking the top 10, but I guess you could make an decent arguement for him there. Johnson has some nice names on his record but they weren't factors when Johnson beat them. Jeffries(who probably rates higher than Johnson) hadn't fought in 6 years when Johnson beat him. Fitzsimmons was in his mid 40's and fighting part-time. Langford was a jr middleweight and McVey and Jeanette were pretty green while Johnson was nearing his prime. Not a lot of credit can go to Johnson for these wins. The wins that count the most for Johnson would be guys like Burns, who was actually a decent enough champ but weighed less than 170 pounds when Johnson beat him. Flynn, Martin, Ferguson, and maybe another guy or two were your typical top 10-15 contednder. Not bad wins but this is about as good as it gets for Johnson. When you add it all up and factor in some his losses I think he is either just in or just out of the top 10. There's no way I'd put him as high as guys like Ali, Louis, Marciano, Lewis and others when judgin by resumes. He never got to face a prime Jeffries which would have been a huge win if he won but he did the same to guys like Langford, Jeanette, and McVey when he was champion.
Kill yourself. Jack Johnson probably single handly caused the colorline to stay in place for years longer than it would have because of how much of a jackass he was.
Yes, definitely in his era but as all other sports boxing has also progressed. But he is one of the finest ATG no doubt.