I strongly disagree. Health is defined in the rules and it does not change. The investigator with the most Health has the highest threshold for taking Damage cards, and this is what they are asking for.
If they meant remaining Health, they would use 'remaining Health'.
Actually you're right. I had missed that part in the rules:
Health and sanity are fixed values. An investigator’s health and
sanity are not reduced when he suffers Damage or Horror.
This, strangely enough, makes it nearly impossible to actually select the character which has the least "remaining health". As health is a fixed value and as such, saying "remaining" health doesn't make any sense.
If you try to say, has suffered the most damage, it doesn't take into account the original health.
So to actually do this, they have to go the long way around and say something like: "The character which can presently take the highest amount of damage without becoming wounded or dying".