Fallout 3: Find and disappoint your father.
Fallout 4: Find and disappoint your son.
Fallout 76: Find and disappoint your audience.
Edit: I forgot about New Vegas.
Fallout New Vegas: Find your attempted killer and be disappointed knowing Obsidian won't make another one because Emil and Todd are salty.
Or... Find your attempted killer and disappoint the NCR, Mr. House, Caesar, or all of them.
Edit 2: Throwback time!!!
Fallout 1: Find your water chip and disappoint your overseer.
Fallout 2: Find the G.E.C.K. and disappoint your village.