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Given the change in technology and tactics over the years, how would a historian go about comparing someone like say Hannibal to a more modern general such as Napoleon?
Well, the short answer is 'they don't.' Moreover, they can't, at least not in a meaningful and rigorous manner, and certainly not in order to 'judge certain figures relative to other time periods.' There are so many ways in which the context, world and experiences of Hannibal are utterly alien to those of a man like Napoleon that it is pointless to attempt to compare the two and expect some kind of meaningful outcome. Even from the strictly militaristic angle - which is itself devoid of the all-important broader societal contexts in which these people lived and fought - there is no meaningful way you can compare the military exploits of a general commanding an army in the Second Punic War with those of a general in the Napoleonic Wars. The tactics, the technology, the command and control, the geography, the numbers of soldiers - everything is different, and any attempt to compare or 'rank' people from such totally different circumstances would immediately devolve into baseless speculation.
Here on /r/AskHistorians we actually have rules against such activity precisely because of the fact that it's impossible to create 'poll-type' discussions of this sort in a meaningful way. Any list - say from Cracked.Com or 'Five most important battles' or 'Five Greatest Generals' or so on is, by its very nature, shoddy history. The criteria by which we might try to rank one general as 'better' than some other general who lived in a totally different world hundreds of years apart are impossible to establish and will inevitably be hugely inconsistent, and as a result any attempt to rank them becomes personal opinion.
Even aside from the impossibility of effectively 'ranking' generals is the pointlessness of it. What do we gain from successfully saying that Hannibal was a better general than Napoleon? Have we learned anything? It might be a fun watch on Mike Duncan, but it's a pointless exercise from the view of a historian attempting to learn more about the world of the past. Even if professional military historians had ways by which they could effectively compare generals from different contexts, they wouldn't bother doing it. They scarcely bother comparing generals from the same conflicts these days (that's a wide net, but you'll hopefully forgive me) because it's a line of thought that lends itself to the deeply flawed Great Man way of thinking. Comparisons of Montgomery and Patton might provide insight into the military structures and organisations in which they operated, and certainly do occur in an academic context, but never with the goal of ranking one 'better' than the other.
In any event, in an academic context, comparisons of Great Men - particularly generals - is generally viewed as an overworked field from a niche area (military history being enormously popular in 'pop' history, but a very small professional field academically). For this and the reasons I've discussed above, you'll find that rankings and comparisons of generals are likely to remain the purview of speculative books and Youtube channels, and not the field of academic history.
If you'd like to read more about the broader themes of speculation, periodisation and poll-types here in the context of AskHistorians, check out our Rules Roundtable series below:
If you have any more questions or concerns, I'd be happy to do my best to help!