you know that almost all applicants will have good academic records and the rest of your application should provide plenty of information on that area anyway so I would suggest focusing on your personal interests and relating them to how they make you a well rounded and balanced potential doctor.
If I remember correctly mine started with 'ever since i can remember i have wanted to be a doctor...'. Another possible way of starting is with a famous quote that means something to you.
I would suggest you make it as personal to you as possible and try not to be too formal. Also get someone like a school's career advisor or head of year/form tutor etc to read through it if you can.
Ranjit (3rd year)