Entirely arbitrary. Once you make a mental list of some sort, you could take the top five, or ten or twenty. It's up to you. Considering that there have been at least 4000 pro HWT boxers since Joe Louis day, I'd say the top twenty (less than half a percent ) might reasonably be described as ATGs.
And then you're still left with what criteria to use and how much to weigh each criteria. Head to head, longevity, quality of opponents, achievement, etc. And when you start in on comparisons of ltwts with Mwts and with Hwts (lb for lb stuff) it can get interesting. Hence the great disparity of views on here on such matters and hence the occasional minor squabble on who ranks 37th on someone's list, when they're clearly no better than 40th on someone else's.