Most weeks at No. 1 is a lesser metric, What counts is number of years at No. 1, and that count is as follows.
Here, I exclude non-contemporary rankings and "period" rankings ("...so-and-so was generally acknowledged as No. 1 from 1942 to 1952.)."
Only include contemporary full rankings (not just a reference to "No. 1", but a ranking of players in order).
For recent decades, use only ATP points, not ITF World Champion (which is a subjective choice).
The ITF World Champion is no more objective than the old ranking lists before the ATP points ranking.
This will hurt Kramer and Gonzales because in those decades some years did not have good ranking lists by contemporaries.
1. Gonzales...11 years
1. Laver...9 years
2. Djokovic...8 years (not including 2013 ITF WC)
3. Tilden...7 years (not including Vines 1933 rankings)
4. Kramer...3 years (Kramer hurts the most by losing "period" rankings, casual references, non-contemporary ranking references)