General, abstract praise -- e.g. that someone has great analytical skills or writes beautifully -- simply won't help in such high-caliber competitions. Such general statements ought to be illustrated with specific examples -- e.g. a description of a specific paper that demonstrates those writing and analytical skills -- in order to show what makes the student in question special in your eyes. Someone reading your letter should easily see how and why you hold the applicant high in your esteem.
Where possible, referees can and should also speak to the candidate’s character and their ability to lead and use their talents to the full.