Too many to mention. Also "Best Goal" can be defined for it's beauty as well as for it's importance.
As I said, there are too many beautiful & important goals out there, but the one that I will always remember, both important and gorgeous, is Zidane's Champions League Final volley vs Bayer Leverkusen.
Not only was it extremely difficult and skillful goal to score but it also won the Champions League trophy for Madrid back in 2002.
Mine's is this impossible shot from the former Brazil international Roberto Carloz!!!!