What is the earliest hook to modify post content?

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When two or more filters are hooked to the same filter hook, the conflict is resolved as follows:

  1. If the filters are hooked with a priority parameter, the filter with a lowest value for the priority will be executed first. If no priority value is supplied, the default value is taken as 10.
  2. If two or more filters are hooked with the same priority, the filter that was hooked first will take priority.

Eg.

add_filter('the_content','filter_1',5);
add_filter('the_content','filter_2');
add_filter('the_content','filter_3');
add_filter('the_content','filter_4',1);

In the above example, filter_4() will be called first, followed by filter_1(). filter_2() and filter_3() have same priority ie. 10. Hence, filter_2() will be called next because sequentially, filter_2() was added before filter_3(). And of course, finally filter_3 is called.

Reference: WordPress Codex Page.