Every argument about the best striker eventually runs into the same obstacle: the candidates play in leagues that never face each other. The comparison requires an assumption about league quality.
Continental competitions provide the only direct evidence
Clubs from different leagues do meet in continental tournaments, and results there are the raw material for any estimate of relative strength.
The sample is small and skewed, because only the strongest clubs from each league qualify and they are not representative of the league as a whole.
An estimate built on those matches therefore says more about the top of each competition than about the mid-table clubs most strikers actually play for.
Player movement gives a second, slower signal
When a forward transfers between leagues his scoring rate before and after provides a data point about the difficulty gap.
Aggregating many such moves produces a rough conversion factor, which is how most public league-strength ratings are constructed.
The weakness is selection: players who move are not typical, and the ones who fail and return quietly are underrepresented in the data.
Style of play distorts the comparison further
Leagues differ in how many shots a match produces, how deep defenses sit and how often transitions occur, independent of overall quality.
A competition with open, stretched games generates more chances for everyone, inflating goal totals without the strikers being better.
Adjusting for chance volume rather than for goals alone removes some of that distortion and usually reshuffles the ranking noticeably.
American comparisons carry an extra complication
The domestic league operates within roster rules that concentrate spending on a few positions, so the quality gap between attackers and defenders can be wide.
A forward facing that distribution encounters a different problem than one in a league where resources are spread evenly across a roster.
Comparisons that treat league strength as a single number miss this, because the relevant question for a striker is the quality of the defenses he faces.
Honest rankings state their assumption out loud
Any list of the best strikers embeds a view about how much a goal in one competition is worth relative to another, whether or not the author says so.
Rankings that publish the adjustment can be argued with productively, because the disagreement becomes about a stated number rather than about taste.
Those that do not are asking the reader to accept an invisible assumption, which is why so many striker debates never reach a conclusion either side accepts.