Which countries offer best for medical studies?

In what comes to the best medical schools in the world, the USA and UK dominate the field, with something like 30+ of the top-50 medical schools in the world being in one of those two countries. Australia and Canada also feature well.

As for 'vacancies', I'm not quite sure what you mean with that - are you referring to job opportunities? In which case, your question should really be where can you actually get a job, as in where are you legally allowed to work, where do you speak the language, etc. - that narrows it down to very few countries, most likely. So just because there may be plenty of 'vacancies' in, say, China, if you don't speak Chinese what good are those vacancies to you?

There is NO way you will remain in a foreign country if you earn your medical degree there. Canada does not accept any foreign medical students. US, Australia, etc, no longer have enough residency positions for all their citizens earning MD degrees in their own countries. Ditto for UK. You MUST return to your country of citizenship immediately upon completion or termination of studies, take your country's medical exams, then apply to hospital intern/residency programs to complete your on-the-job practical training & any specialization.

No shortages of doctors anymore except in countries like India, Pakistan, Bangladesh, some African countries. A medical degree is not a path to immigration anywhere anymore. You will ONLY practice in your country of citizenship.