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Kizuna Ai, and it's not even close. Any other answer is objectively wrong. She was the first Vtuber and spearhead of the whole Vtubing moment. She was leaps and bounds ahead of any competition, all her competitors used to show deference to her and many, including Tokino Sora, started out as blatant clones.
Then the Four Kizuna Ai happened. It was an unmitigated disaster. They created a Chinese Ai in the hopes of catching the bugman audience but it turns out the Chinks wanted to watch actual Kizuna Ai, they didn't want a Chinese speaking Ai any more than the rest of the fans. The other two Kizuna's gradually began to take more prominence over the original, she began to appear in fewer and fewer videos. To this day, there are still people who think the original VA got replaced, years later. Eventually they spun off the two Kizuna Ais into their own channel with their own models, and one of them graduated shortly after. The other might still be around I'm not sure. Interest in Kizuna Ai channel dropped off a fucking cliff, because people were there to see Kizuna Ai, not two newcomers using the model and a Chink. At this same time, the format of Vtubing shifted from short videos to long form content like we see now. Hololive was at the forefront of this trend, and Tokino Sora, the Ai Clone that used to have to beg for scraps to show up in Kizuna's event, helped lead a plethora of new talent to the forefront. Upd8 was crippled by this, to the point where the original VA had to come make a statement but it was too little too late. Like I said, there are still people who think she was replaced and that is solely due to bad communication on the part of upd8. Many of their talents left, including the one that would later become Omaru Polka.
Kizuna Ai was a shadow of her former self, losing the race to 3 million to Gura, despite having FIVE YEARS and 2.5 million sub head start. She eventually held a concert and went on "indefinite hiatus" but was for all intents and purposes a graduation.
To this day, the choice to make "The 4 Kizuna Ai" video might be one of the worst fucking business decisions in the history of money. They were the leader and pioneer at the forefront of vtubing and they shot themselves in the foot. The confusion and disinterest following this, as well as the change to more live streaming and zatsudan content, sent Kizuna Ai into obscurity of an industry she basically created and lead for years.