Who was looked as the most dominant fighter by the public entering their bout they lost? Foreman pre-Rumble in the Jungle Tyson pre-Douglas RJJ pre-Tarver
Back then, the public believed that Foreman would literally kill Ali in Africa. It has to be Big George.
Without a doubt. As good as Foreman looked in his prime, there was always that division full of great names to challenge him. Tyson had no looking remotely like they could give him a hard fight, never mind actually beat him.
Who was more perceived as unbeatable and who was the closest to being unbeatable are two different questions.
Well, it's actually a pretty easy question if you frame it as before they lost the Ali, Douglas and Tarver fights, respectively. Roy Jones nearly lost to Tarver in their first fight. That's why they rematched. Tarver had no fear of Jones. They knew each other as amateurs. Tarver interrupted Roy's post-fight press conference folliwing the Ruiz fight demanding a shot. He claimed Jones was ducking him. They were supposed to fight earlier, but Jones insisted Tarver beat Eric Harding first, and Tarver suffered a broken jaw in his only loss. Tarver beat Harding in a return and said Jones couldn't duck him anymore. Many thought Tarver won the first meeting with Jones. People gave Tarver a good chance in the return. Roy Jones and Tarver were both 35 but Tarver seemed on the way up, while Jones (after Tarver 1) seemed to be sliding. While it was an upset, what was more shocking was "how" Tarver won (by brutal KO). But the win itself wasn't a total surprise. In his first fight with Tarver, Jones already looked vulnerable. George was an 8-1 favorite (which is a big favorite) over Ali. Foreman had destroyed the only two guys who had beaten Ali (Frazier and Norton) in just two rounds. But there was a month-long delay when Foreman suffered a cut just days before he and Ali were supposed to meet. Both fighters were stuck in Africa, because the government (the dictator at the time) took their passports and wouldn't allow them to leave (fearing they'd never return). And during that month, Foreman couldn't spar much and Ali reveled in all the fan support. So, by the time the bell rang, people were very anxious. They still thought George would win, but it had become a more surreal situation where it seemed anythign could happen. Ali was 32 years old and had a 44-2 record. He was already considered one of the best heavyweights who had ever lived. Foreman was 25 years old and 40-0 and seemed like the future. After George beat Ali and Frazier, it would be an official "changing of the guard" that would put an end to the 1960s heavyweights and signal a new era had officially begun. But Foreman couldn't do it. In 1990, Tyson had beaten all the former champs (Spinks, Holmes, Thomas, Tubbs, Berbick, Smith) that had immediately preceeded him. He'd destroyed his #1 contenders - Bruno and Williams. Williams didn't last a round. Tokyo was offering Tyson like $10 million to come back and fight anyone. Don King scrolled through his rolodex of boxers he promoted and Buster seemed like an easy two-round KO victim. The first time King promoted a Douglas fight King put him in with David Bey in Bey's pro debut. You don't want your amateur star to lose in his pro debut, so you don't match him tough. King didn't think much of Buster. So he matched David Bey with Buster and Bey won. From then on, King would occasionally promote Buster if he wanted one of his fighters to score a win over a decent opponent. Or he'd use him way down buried on the undercard to fill out a show. Over the years, Buster pulled a couple upsets (over Greg Page and Tex Cobb) and King got him an IBF ranking and was able to present him as an opponent for the unbeaten Tony Tucker when he fought for a vacant belt. Buster lost. Douglas scored a couple more upsets on Tyson undercards (against Mike Williams and Berbick), and Buster was on the same fighting schedule as Tyson. So he was an easy opponent to plug in for Tyson's second Tokyo appearance. But Buster was no Ali or even Antonio Tarver. He was a 42-1 underdog. He had some losses to Mike "The Giant" White, Bey, Jesse Ferguson and Tucker. Buster was "just a guy." Tyson was 23 years old and was 37-0. Held all the belts. Had beaten all the former champs and the #1 contenders. He was "The Man." Douglas-Tyson was the biggest upset in the 20th century for a reason. Tyson was the unbeaten monster.
This is a very opinionated post that has no correct answer. One would have to have well remembered each of these times in history when they happened (mid 70s, late 80s, early 2000s). I was not alive for Foreman or Tyson, and was barely alive for Jones-Tarver 1, so I have no answer.