Inoue. Short amateur career, age, and crushing dominance over Adrien Hernandez puts him past Lomachenko. Inoue couldn't legally drink in the United States one year ago.
In terms of greatness after first 6 fights: 1. Muangsurin 2. Inoue 3. Lomachenko EDIT: I forgot Sahaprom.
You don't rate Sahaprom up there?. Chuvatana and Konadu are easily the two best fighters any of these guys fought in their first 6 fights. The Veeraphol-Daorung fight is a classic back and forth chessmatch between two quality technicians, and while Konadu stopped him in the second, it was a shootout where Nana had been on the floor earlier.
Naoya Inoue definitely gets the nod with his wins over #1 ranked Adrian Hernandez as well as Taguchi. Pete Rademacher was the Olympic Gold medalist who fought champion Floyd Patterson in his debut. He also followed that up with Zora Folley in his second pro bout. Perhaps Muangsurin and Sahaprom. Anybody who faced a legendary fighter in their first 6 bouts, which there are literally countless, would have faced better competition in their first 6 bouts. For example, Joe Jeannette fought Jack Johnson in his 4th ever fight.
Maskaevs debut opponent Alexander Miroshnichenko is one of the best debuts that I can think of. Top class amateur with 21 pro fights under his bet and no losses. Maskaev really took no short cuts in his career.
Just on paper. De facto, it was the same type of a journeyman as Chonlatarn Piriyapinyo, Loma's 4th opponent, is. A good record, but no even quarter-credible wins. However, if you want me to name any better debut, I would need to give it a deep thought and hit the google, I'll admit that.