I dunno whether it's the absolute best Batman story of all time or not (though its definitely a contender) but I would go as far as to call it the foundational text of the contemporary Batman mythos and franchise.
Almost everything that has come afterward has been inspired either tonally, or conceptually, or straight-up plot-wise, by Year One in some measure.
Year One is the reason why Batman in the dark grey and black suit with the plain black bat is now the dominant look of the character. It's the reason why Gotham is more often than not depicted as an urban hellscape drowning in organized crime and corruption. Its the reason why we think of Jim Gordon today as not just the Commissioner who needs to pick up the Batphone to solve weird jewel thefts but rather as a crusader and Batman's partner in his own right. It might be the reason why the idea of Batman as a 'vigilante' at odds (if not outright at war with) the cops is such a popular idea. Or why Batman working solo became the 'default' setting of the character in most adaptations and indeed in many of the comics themselves.