We all love Mario Kart 64, but that's largely due to nostalgiavision. Objectively, it's probably the weakest in the series
I'd say Metroid, but one misstep does not a mishandled franchise make. There's just a lot of trepidation in that fandom about what they'll do next, or more scarily still, when they'll do it. The 1994-2002 gap weighs on our minds.
Monster Hunter: pissing me off since 2010.