It would be easy to say that MC best album is Butterfly. The songwriting, the production of the time, the vocal performance especially for the layerings.
But I think I would like to say that both Daydream and The Emancipation of Mimi are two of her most complete albums too. The entire concept of Daydream, the aura it exudes to this day, it felt timeless especially for an album that was released in 1995. Mimi album was exceptionally done, though far from a "most personal body of work" tagline. It's served its commercial purpose, while also gave us songs in the highest quality like Stay the Night and Your Girl, not to mention some Neptunes' works there too.
Charmbracelet is also noticeable for its glossy production and the last time we ever heard top-notch lyricism, until Memoirs of an Imperfect Angel was released. From Mimi to the Einstein album, her lyrics weren't that extraordinary and she received lots of negative feedback from music critics that often said MC was trying hard to cater to the audience of that decade.
All of her albums prior to Music Box were cleverly written, but as usual, there were some missed connection when it comes to the technique of storytelling, which from Music Box onward would never be a problem anymore for her. Music Box felt too mature, not to mention that the album gave the vibe of truly Adult Contemporary which was kind of... boring. But it's still an exceptional album from her in those first years of her career.
I think when it comes to Mariah, it would be so much more difficult to deem a work of hers as her best because there will be a lot to be considered. From lyrical standpoint, her melodies, the vocal performances, some vocal tricks a.k.a. adlibs, the production of each albums, etc.
I think Butterfly remains her "perfect" album, because it still gives me the balance of everything. But It's just me, the thing that appeals to me doesn't mean that it would appeal to a lot of other people. Although I have to say that her Prince cover is straight up unnecessary and doesn't appeal at all, lucky for her that the entire color of the album still made her The Beautiful Ones sounded somewhat in-concept/in-tune. I like her house mix of Butterfly, although I can live without it because the ballad Butterfly is just... the whole entire universe made into a song. That ballad simply gives me the chills and I live for it, every time.