When Yuuki was with Kaname…
Yuuki couldn’t last one year apart from Zero. She stopped smiling from the heart, she was quite literally being driven mad with starvation from being unfulfilled with the love she and her fiance shared, and even when she made declarations and promises to be devoted, she was always drawn right back to Zero.
However, when Yuuki was with Zero…
Yuuki could live over a thousand years without Kaname. With Zero by her side, she got her smile back and maintained it for thousands of years, she didn’t starve… for thousands of years [lol I’ll never get tired of that phrase], she was fulfilled with their love and so did not suffer the fate of a starving vampire, and finally, Yuuki could’ve easily went to slumber to wait for Kaname, but she didn’t and chose to live life with, and love, Zero.
Yuuki would, and did, die for Kaname.
But Yuuki would, and did, live to be in love, and actually experience it, with Zero.
Since the beginning of second arc, Kaname has been Yuuki’s guilt. She even went so far as to wish that she was never born because of how deeply tied by guilt she was to him. I don’t deny she loved him, but I don’t believe her love for him was as pure and strong as what she felt for Zero.
Zero has always been Yuuki’s happiness. He was able to take from her that seed of sorrow Kaname planted in her, and bring her happiness for thousands of years. It is the ideal, and what Yuuki always desired [Haruka x Juuri].
The differences in how she loved both is stark. Yume is filled with questionable darkness, but Zeki… I favor the Zero and Yuuki love because it brings together everything that is positive and endearing about love, and makes it last for thousands of years.