The Anime Lad
The last of humanity lives inside three concentric walls because giant naked people eat anyone outside them. Nobody knows where the giants came from, why they eat us, or what's in the basement of one particular house. Episode one gives one boy every reason in the world to go find out. The show then answers every one of those questions, years later, in order, like it had them written down the whole time.
Super Eyepatch Fox
Three walls, the last humans inside. Outside, giants that eat people they don't need to eat. Episode one, the outer wall falls. A boy loses everything and enlists. Then the questions start. Who built the walls. Why giants exist. Every answer costs someone.
Gigguku
Okay so it starts as a monster survival show, humanity in walls, giants outside, and that alone would've been enough. But then it keeps opening doors. Military drama, political thriller, war story, each one bigger than the last, and the mysteries from episode one keep paying off the whole way. I marathoned the first season in two nights and I have never fully recovered. Start it before someone spoils it for you.
Father's Basement
The first season is rough in places, the pacing wobbles and the lead spends a lot of it yelling. I want that on record, because everything after is one of the most tightly plotted stories the medium has produced. The reveals recontextualize entire seasons, the war story it grows into has real moral weight, and the foreshadowing will send you back to episode one frame-hunting. It won me over completely and I came in armed.