Today I'm recommending Fate/Type Redline by author Keikenchi and artist Hirano Ryouji.
So what happens if you make a gag series based on Fate/Stay Night, but then it got so popular, and then became a serious series? You get Fate/Type Redline. Our protagonist goes back in time to help his grandmother compete in a Holy Grail war between mages of the axis powers. As he travels back in time to help his grandmother when she was his age, he gains the assistance of a powerful servant, Saber Okita Souji, and they battle against oppressive forces, not only to go back to his time, but to save his grandmother from the impending doom.
Honestly, this series is really good even if you have no prior Fate knowledge. It doesn't require any knowledge from Fate/Stay Night or any other spinoffs, you just basically need to know it's a battle royal between mages, the masters and their heroic spirits, the servants. You'll meet an insane ensemble of characters from Japanese history such as Sakamoto Ryouma, Mori Nagayoshi, and even Oda Nobunaga, along with some other interesting characters.
Highly recommend, and yes I know this is more than I usually type, but that's because I love Fate/Grand Order.