#4 Kazuchika Okada
Kazuchika Okada is considered to be one of the best wrestlers in the world right now. He had several great matches with the likes of Hiroshi Tanahashi, Tomohiro Ishii, Katsuyori Shibata, AJ Styles, Shinsuke Nakamura, Prince Devitt (Finn Balor), Tetsuya Naito, and Kenny Omega.
In fact, his match with Omega at Dominion 2018 got seven stars, the highest rating ever awarded by Dave Meltzer.
Kazuchika Okada held the IWGP Heavyweight Championship for 720 days, and he is considered one of New Japan's 'Ace'. Readers of the Wrestling Observer Newsletter voted Okada as 'The Most Outstanding Wrestler of the Year' and 'Wrestler of The Year' in 2017 and was also ranked No.1 in Pro Wrestling Illustrated's 'Top 500 Wrestlers' of 2017. There is no denying that Kazuchika Okada is one of wrestling's biggest star not signed by WWE and it should stay this way
Kazuchika Okada has a very good chance of succeeding in WWE, but certain problems can prevent Okada from reaching the level he could in WWE.
Besides facing the issue of not being fluent in English, Okada's typical match has him start slow and then pick up the pace during the finishing stretch, which may cause some casual fans to lose interest in his matches.
And besides, Kazuchika Okada still has a lot of things to do in New Japan Pro Wrestling. Ever since returning to New Japan from an excursion, Okada has yet to hold any other championship belts besides the IWGP Heavyweight Championship. He can always contend for the IWGP Intercontinental Championship.
There are opponents that Okada never faced (Like Chris Jericho). Okada also needs to help make New Japan bigger than it is since he is the company's 'Ace'.
Okada could succeed in WWE should he decide to work for them. He just doesn't need to go there.