Probably Phoenix Wright since he's such an icon of gaming now and such a huge deal and some people have desperately wanted him since Brawl was announced. Plus like... nearly all of us who play Smash super passionately and often have played Ace Attorney games from now, and most of us love them.
And despite being a bottom tier, his Ultimate MvC3 appearance and moveset was great.
I think Crash and Rayman are close though, although honestly of those Video Game Generation 5 (Game Boy, PS1, N64, Sega Saturn) debuting characters, Cloud and Snake still overshadows both of them due to just the legacy of Final Fantasy VII/Final Fantasy in general and Metal Gear Solid 1/Metal Gear Solid in general, and they were already in Smash, and their series remain huge while Crash is just now coming back, and Rayman has only been "back" for half a decade (with really only 2 titles).
Phoenix Wright would be the sole Gen 6 (GameCube/PS2/Xbox/Dreamcast/GBA) debuting 3rd party character, and he's probably the biggest handheld console 3rd party star ever. He's also really popular with mobile device gaming fans. Plus so many Phoenix Wright jokes and references are super popular in pop culture and social media.
Master Chief could bring hype yeah, but like... Halo (while IMO coming off a solid game in Halo 5) is not the huge pop culture and box office phenom it was a in the 00's. Crash is finally back, but it hasn't really resonated like the old days. Tomb Raider (Lara Croft) and Rayman are better than ever, but the "newness" is gone. Bomberman was always super niche and likely will never be anything but that. Banjo had 2 games that were fairly big and then he's basically been dead.
Ace Attorney still is gigantic. The initial popularity still remains, and Phoenix Wright is as popular as ever.