I honestly never understood why so many people consider YI1 to be the pinnacle of the Yoshi series. I mean, yes, YI1 is a fantastic game. But are the sequels really any less fantastic? YI2 was basically everything YI1 was but more, and maybe some people don't like the complications that the baby switching throws in, but I thought it was a pretty neat mechanic. YNI is admittedly a bit lazy in how heavily it borrows from YI1, but taken on its own merits, it really does everything that the original did and does it better (plus the new art style was absolutely beautiful, fight me). I also disagree with the notion that the SNES version of YI1 is better than the GBA remake, because it's not.
However, in my eyes, the true pinnacle of the main Yoshi series is the Woolly/Crafted World line of games. I haven't played Crafted yet beyond the demo so I can't definitively determine which is better, but I absolutely adore Woolly World and it easily surpasses all four of its predecessors. Even if it plays a bit differently and is no longer focus on saving babies, it's largely true to the established formula while still throwing in a good number of its own unique tricks. Crafted World looks like a worthy successor and I'm looking forward to playing that.
Story is the weakest of the six main Yoshi games in my eyes, but it's by no means a bad game and I enjoyed its more open and non-linear level design. I'd probably like it a bit more if it were longer.