Just to name quite a few, spoiler tagging them in case some haven't seen this you've been warned.
Medusa vs Evil Saber {hands down the most intense battle ever in Fate}
Majin Buu vs Majin Vegeta {self destruction}
Head Captain Yamamoto vs Quincy King Yhwach full fight {seeing this animated after a decade was worth it}
Edward vs Father {brotherhood}
Alucard vs Walter {Hellsing ultimate}
No name vs Luo Lang {Sword of the Stranger}
Majin Buu vs Majin Vegeta {self destruction}
Head Captain Yamamoto vs Quincy King Yhwach full fight {seeing this animated after a decade was worth it}
Edward vs Father {brotherhood}
Alucard vs Walter {Hellsing ultimate}
No name vs Luo Lang {Sword of the Stranger}