Reagan was more important in the fall of the Soviet Union than Gorbachev.
The pivitol moment was the Reagan/Gorbachev Summit in Iceland in 1986. In Reykjavík, Gorbachev offered to reduce the Soviet Union's stockpile of nuclear weapons to ZERO - only if Reagan would agree to limit testing of his SDI program to the "laboratory."
Reagan refused.
Gorbachev was desperate and Reagan knew it.
At that moment, NO OTHER PRESIDENT who served during the Cold War would have done what Reagan did.
At Reykjavík, Reagan broke away from the "mutually-assured-destruction" mentality, put America first, and left the Soviets out in the cold.
From that moment on, Gorbachev knew he could not win the Cold War.
Reagan was the catalyst. But the trifecta was completed with Thatcher and JPII. Those three powerhouses were instrumental in the breakdown of the old Communist-style Iron Curtain that had covered Europe and threatened America for 40years.
JPII was probably most responsible cracking the grip of Communism in Eastern Europe, especially his homeland of Poland, where the Solidarity Movement started. That's why the Soviets (most likely) hired that Turkish patsy to assassinate him.
Thatcher was the outspoken Conservative stallwart who kept the rest of NATO in line with the US and Reagan. Behind the scenes, she showed the Soviets that NATO and Britain especially was firmly behind Reagan - even if outwardly most of NATO were just whimpy, finicky socialists at heart.
But I'd say it was Reagan who was most instrumental in accelerating the collapse of the Soviet Union.
Gorbachev clearly should get the credit for having the good sense to do it as peacefully as it was done. For keeping the hardliners from taking over and for ensuring a "peaceful" transition, Gorbachev was a uniquely rational and wise Soviet leader at a time when reason and sanity was MOST needed in the Kremlin.