Puyo Puyo 20th Anniversary. (Chronicle is a close second)
Puyo Puyo Tsu is the greatest competitive puzzle game ever made. Such a simple set of mechanics gives way to an incredible amount of depth. I think its greatest strength relative to the rest of the genre is how much importance it places on actually paying attention to and adapting to your opponent. Some of my favorite other puzzle games are guilty of feeling more like a game I play adjacent to my opponent rather than against them, and I'll give them a pass if the core gameplay loop is fun enough, but I consider Tsu king of the genre for having the most true versus in its versus mode.
But Tsu's skill curve is terrifyingly impenetrable for beginners, it's one of the hardest competitive puzzle games to learn. Just understanding how to make chains is extremely daunting, and that is but the tip of the iceberg. Paying attention to what your opponent is up to while still being able to concentrate on what you're doing is an order of magnitude harder, and that's kind of where the real game begins.
20th shines by being the most comprehensive package full of additional content for players of all skill levels alongside the classic Tsu ruleset. There's a whopping 20 different game modes to play around in, many of which are much more immediately fun for a beginner to pick up, get hooked on, and hopefully enjoy the game enough to want to eventually learn to scale the mountain that is Tsu later.
Sadly, this game never got released in the west, and none of the games that have come anywhere close to it. And I think that's a large part of why the series is struggling to gain any kind of recognition in the west, we've never seen the best of what it has to offer.