#3 Andrade
Andrade became a main event star in NXT. In the beginning, he wasn't considered as a very promising talent, but once Zelina Vega became his manager, Andrade climbed higher up the card, and ultimately won the NXT Championship.
When he arrived on the SmackDown, he was not given much screen time; then he had thrilling matches with Rey Mysterio at the start of 2019, causing speculation that both men could face each other in a Mask vs. Hair match at WrestleMania 35.
Mysterio ended up facing United States Champion Samoa Joe instead; Andrade participated in the Andre The Giant Memorial Battle Royal and lost. Obviously, he would be unhappy about it and could consider quitting the company.
#2 and #1 Rusev and Shinsuke Nakamura
Since Rusev and Shinsuke Nakamura are working as a tag team, I will be documenting their displeasure with the company together. Both used to work as individuals, then three months ago they were booked to work as a tandem after ending their United States title feud.
Rusev has spoken about his discontent with WWE in many interviews, saying that the company has mishandled his character on various occasions - in particular being against the split of Rusev Day. Even his wife Lana has been unhappy about her WWE career.
As for Shinsuke Nakamura, he had a brilliant career in NXT, but his character was underutilized on the main roster. During his rivalry with AJ Styles, and ever since, Nakamura's booking has been his detriment.
Both Rusev and Nakamura could have become top babyfaces in WWE, because they were beloved by the fans. But WWE failed to utilize their potential and turned them heel in desperation. These two are likely considering a career outside the WWE.