This needs to be answered!
It's Katanagatari, as answered before.
And for anyone wondering why it's a troll:
For the first 3 episodes of the series, Sabi Hakuehi, the white swordsman in that gif, is hyped as the strongest swordsman in Japan. The preview for episode 4 shows that gif of their epic fight. Episode 4 happens and the fight is never shown, focusing on a completely different character, only switching back to the main characters at the end as they describe how totally awesome the fight was.