There were 3 routes that were filled with the darkness that Oliver has to fight in the series and each within the seasons 8-10. But which of the 3 was the most fun?
1. In season 8, Oliver's darkness is almost embraced as he seeks revenge on the Luthor's for killing his parents, murders Lex Luthor, and tries to have everyone believe including himself that killing is worth it if it justifies the means in the end.
2. In season 9, Oliver's darkness had turned him into a drug, drinking loser for the first 5 episodes that made him not care about his life. He got Lois turned into a zombie, almost caused the death's of countless people because of his dark deeds in turning Toyman into a wanted man, and more.
3. In season 10, Oliver's darkness returned to him when he lost Chloe and not knowing where she is. He started to give up on himself more as he told the world he was Green Arrow and did much into trying to find her. He also was corrupted to the darkness completely by Darkseid after almost killing Desaad thinking he killed Chloe. He was then battling that darkness in him until he became a warrior of Darkseid's in Prophecy.
For me I loved his journey in Season 10 more because his darkness was more relevant to the story but on the other hand his season 8 dark side was also fun leading him to kill Lex and leading clark to completely realizing that killing is never the answer. It also helped me like the doomsday story a little better. In season 10, he was the bad guy in the Finale for half of it and his turning to darkness was a great story with zod and kara.