What country has the best quality of diamond?

'Best quality' is a judgement, and you are the judge.

Best quality may imply highest investment value. Any diamond, regardless of its country of origin, with top qualities in cut, colour, clarity and carat weight, will generally 'hold' or 'improve' its investment value over time.

An infrequently used measurement of diamond is the typeof diamond, below. Type IIa diamonds named in the reference come from Australia, South Africa and probably India.

Being unaware of human political boundaries, Mother Nature formed the 'best quality' diamonds deep within the earth's mantle probably all over the planet.