Hello,
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Both are best.
Both the countries comes under top 10 countries for studying MBBS.
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But when it comes to comparison among the best then Ukraine is best.
Because Ukraine is 4th best country for MBBS where as Russia is 6th best country.
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So Ukraine comes under 4th for its education facilities and placements opportunities.
So going for Ukraine will be best.
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Hope your question got cleared.
Thank you