There was indeed a less than enthusiastic response by many heads of state and heads of government. Analyzing every single one could take an entire book.

In brief: After Germany was reunited, the Prime Minister of Italy Giulio Andreotti remarked, "I love Germany so much I liked it better when there were two of them." Andreotti's phrase is a reference to an earlier remark by french poet François Mauriac, who after the partition of Germany after the Second World War remarked, "I love Germany so much I'm happy that there are now two of them."

This article in the International Policy Digest very briefly goes over Andreotti's political career.

Margaret Thatcher also went on record saying that she was not favorable to German reunification, and later held that those who supported reunification didn't believe it would actually happen (a bit like the Brexit?). The French president Francois Mitterand, on the other hand, although affirming he was against German reunification in official communications with Gorbachev in those same communications affirmed that he believed unification was inevitable.

Indeed, the only major power (apart from, of course, West Germany) that was unequivocally in favor of German Reunification was the United States.