Johnson IMO, & I don't think it's even close. McCallum was a great fighter, but I think he's a touch overrated on the whole. he was a very well-rounded fighter but wasn't really outstanding in any single area IMO. he also came up short in too many big moments for my liking. by contrast, Johnson had an ATG resume spanning LHW-HW w/ wins over 1 of the top 5 best ever in Charles & 1 of the top 15 best ever in Moore.
It's close. I'd pip it for Johnson personally but hardly argue it back and forth. His win over Charles nips it and he beat a top 5 heavyweight as well among numerous other career exploits.