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My personal favourite was Round 5 because it was the first full round I had ever played. The powerblock techniques of Legion and Fury were in their infancy (or so it seemed to me) and the cluster alliance politics seemed really important
It was the time when I got to know many of the people who I've worked with over the following rounds and many of them have become really good friends both online and in real life.
If anyone wants to comment further on their reasons for their favourite round please post here.