"Best" is a relative term. One can discuss one's "best" tooth extraction, but it is still a tooth extraction. One could find an example of anything and call it the "best" version of that thing. I don't like Yandere characters.
Yandere characters in anime and manga are annoying. The word itself is used for psychotically obsessed (usually female...like 99% female) characters who seem normal (well...maybe overly devotional and self-sacrificing, but that side can be a little sweet at times--one characteristic of a yandere is that they have very distinct sides of their personality, the term actually suggests "sweet outside-crazy inside"), until they are in the vicinity of the one they love. These characters would literally do ANYTHING for love. Axe-murdering? Sure. Dramatic suicide attempts? Absolutely. General loony toons? To the extreme.
A yandere character woud conceivably even kill the object of their affection in a classic "If I can't have you no one can" sort of Mark Wahlberg in Fear dealie. It's all very Glenn Close, all very "I boiled your bunny," but a yandere character might even take it a step further. "I boiled your bunny, but I am also dressed as one, please pet and love me?" They're really screwed up, and they tend to switch so quickly from cute-cute to ultra-violent that they seem hyperactive and ridiculous.
So, the "best" yandere character? That's a pretty lame topic. I pick Kagura Sohma from Fruits Basket .
Poor Kyo. It isn't easy being the object of a yandere's obsession, especially if that yandere is the zodiac incarnation of a boar. She might be little, but she is powerful!