As of today, leaderboard is: LaMar Freeman "Cedar City Bomber" Clark - American heavyweight, ran himself to 42-0 (41), winning his pro debut on points and then scoring forty-one consecutive stoppages before stepping up in class and getting exposed thrice over (by Bartolo Soni, Pete Rademacher, and Muhammad Ali). Of those 41 men that he knocked out however, 27 were making their pro debut - making this streak, if numerically significant in the record books, maybe not exactly the stuff of legend. Lots of these stiffs would make most modern Thai fighters with their padded forty-odd-and-0 records blush. Donald Wayne "The Man of Steel / Don" Steele - American heavyweight. Technically speaking, the stocky trucker knocked out 42 men in a row (which ought to put him tied with Clark) but his third match, versus David Nelson, was unsanctioned - and thus goes down on his official record as "No Contest". Starting the count then with his subsequent KO victory after Nelson, his streak hit 39 before he ran afoul of Danish puncher and Olympic bronze medalist Brian Nielsen. Only nine of the 39 were debutants, an improvement over Clark's percentage (in fact, proportionately, only a third as many). William McKinley "Blackjack Billy" Fox - American light heavyweight, went into his challenge of Gus Lesnevich with a 36-0 record. All of those victories were by KO. While not as dire as Clark's opposition, these 36 were at best fringe contenders. Lesnevich ultimately showed the difference in class, dominating all but one round and stopping Fox in 10. A year later he would blitz Fox in a questionably-earned rematch (with LaMotta going in the proverbial tank in Fox's directly preceding bout). Rodolfo "El Gato" González - Mexican lightweight, slugged his way to a fearsome 33-0 (33) on the domestic scene. This was an entire decade before González - incidentally José Becerra's cousin, and sparring partner of Mando Ramos and Carlos Palomino - would retire Eudibiel Guillén "Chango Carmona" Chapín to claim the WBC lightweight championship of the world. Of his sequential first 33 victims, granted, 24 of them were making their pro debut - but on balance I'm willing to err on the side of putting a little more stock in the merits and toughness of novices from the mean streets of Michoacán than in Clark's fodder over the same time period ('59-60) in Utah. Wilfredo "Bazooka" Gómez Rivera - Puerto Rican super bantamweight, debuted with a draw but over the next nearly 7 years would halt every contest inside schedule. A total of 32 destructive knockouts, encompassing the majority of his reign at 122lbs. If not for the intervention of another ATG by the name of Salvador Sánchez Narváez, who knows how much further than 32 that number may have swelled to - certainly, the following year, Bazooka had no problem picking up where he left off. He would stop eight consecutive foes (including four more super bantamweight title defenses) between his unsuccessful WBC featherweight title challenge against Sánchez and his successful one against Juan LaPorte three years later. Deontay Leshun "The Bronze Bomber" Wilder" - American heavyweight, tied with Gómez for 5th place with 32 knockouts in a row. Bermane Stiverne, in their first meeting, would extend him the full twelve round distance to prevent Wilder from breaking Bazooka's streak - not that Wilder minded, as he got his first world championship in the bargain with his UD victory. Much like Fox's, the opposition faced by Wilder before his first world title shot is somewhere in that vastness that soars high above LaMar Clark's quantity-over-quality 42-0 but still doesn't occupy the same atmospheres as eventual three division champ Gómez's legacy-making 32. By far the most impressive there IMO, in terms of substance, is Gómez's. If you want to exclude him and Clark on the basis of their debuts (and Steele due to his unsanctioned knockout turned NC) and look more narrowly at the longest KO streaks to start a career, then it's down to Fox followed by González and then Wilder as the "Big Three" worth chasing. We may or may not see any of the above displaced in our lifetimes. I'm not holding my breath for anyone to even crack thirty in a row anytime soon. Currently the longest kayo streaks in the sport are: Alessandro Riguccini, 20 Nelson Hysa, 17 Naoya Inoue & Dayán González, 11 Andy Hiraoka, Luqmaan Patel, Bek Nurmaganbet, and Subaru Murata, 10 Armando Martínez Rabi, Jadier Herrera, Alejandro Luis Silva, Abdullah Mason, Mikito Nakano, and Enriko Moses Itauma, 9 Riku Masuda, Kemahl Russell, and Joshuah Lupia, 8 Hovhannes Bachkov, Michał Cieślak, and Osleys Iglesias Estrada, 7 Isaac Lucero, Emiliano Vargas, Bekman Soilybayev, Ernesto Mercado, and John Mannu, 6 Euri Cedeno, Jack Turner, Frederick Gleluo Kiwitt, Riku Kunimoto, Daud Cino Yordan, Hayato Tsutsumi, Miguel Ángel Esparza Cruz, Masamichi Satô, and Ağıt Kabayel, 5 dozens-way tie at 4 (Gervonta Davis, btw, managed to sandwich 16 between his UD wins over Germán Meraz and Isaac Cruz...closest I can find in recent memory to bettering Hysa or Riguccini) That's from everyone in the BoxRec top 500 p4p (presumably including just about anybody world-ranked...or at least it sure as hell ought to) - just 34 men have stoppage streaks of more than 4 in a row. Everybody else, if they're not coming off a loss or a points victory or draw or No Contest, has between 1-4. Subtract the nine-way tie for 9th place, and you have just 25 boxers with 6 or more. Just eight in double-digits - and of those, only Naoya Inoue is facing world class competition. Er, he is sometimes, anyway. Riguccini and Hysa might pump their numbers up into the mid or even high twenties, but neither is particularly young. They'd have to really scrape the bottom of the barrel to pad themselves up to Wilder/Gómez territory. Inoue is himself 32 years old, and it has taken him six years to build up an 11-0 streak. He would need to triple it to bump anybody off the all-time list. Great as he is, don't see that happening. Especially if he keeps moving up in weight and facing ranked contenders, eventually somebody's going to become the first since Donaire to last 12 (if maybe not beat him).
GGG's knockout streak against top 10 contenders and other world champions was probably the best I have witnessed live.
I wonder how many boxers had two separate streaks of at least 20 straight stoppages like Hall of Famer Carlos Zarate did. He stopped his first 23 opponents before the unremarkable Victor Ramirez (lifetime record of 11-12-2) went the 10 round distance. Zarate then ran off a streak of 28 in a row before he got himself kayoed by fellow Hall of Famer Wilfredo Gomez.
Tyrone Brunson's streak was 19-0, all victories by 1st round stoppage (broken when he drew with Antonio Soriano). Seven years later his KO1 streak would be broken by Yemeni strawweight Ali Raymi. While the legitimacy of much of Raymi's career has long been questioned, he would finish with a record of 25-0 - all twenty-five victories by stoppage - before he was killed. The first 21 came in the 1st round - meaning he only broke Brunson's streak by a single fight. The closest anyone has come to breaking that KO1 streak since is Edgar Berlanga, going 16-0 with sixteen stopped inside a round before Demond Nicholson spoiled the party by stretching him eight.