Strictly speaking the post is not about 10 best cosmonauts but about cosmonauts involved in the first 10 successful Soviet space flights. Starting from the seventh the flights were not more individual so you'll find here more than 10 persons. Let's start.
1. Yuri Gagarin
First flight of a man into the space at the height of 327 kilometers lasting in total 108 minutes that have changed his life and life of our planet.