The above examples would make for a "big-wig" letter, but they are not needed for a "big-impact" letter.
In general, the more senior the author will mean someone reading it will recognize or know the person. The author should be from an academic institution or have an academic type practice. The more the author has mentored medical students, the better. The more letters of rec written by the author, the more powerful the letter (some letter writers write on behalf of several students each year- it does not take much to figure out which of the applicants they think is best).
There are many "code phrases" make for the most powerful letters. Here are some examples: "In my 25 years of mentoring hundreds of medical students, I would place this student in the top 5 of all students I have encountered." "Our residency program is consistently considered one of the best - our program would be lucky to have this person and I will be sad to see them go elsewhere."
In summary, choose experienced letter writers who know you well. You do not need the biggest name.
Finally, a big wig in ophthalmology may have little impact in a medical school admission committee room. Do you believe the average family practice doctor knows who david parke is? - I don't think so. Again, experience letter writers who know you well will be your best approach – often times these are the "big wigs"