Douglas Tyson was a huge upset but it was a downslide Tyson. Leonard was younger in his career vs Duran, and Ali was just past his absolute best vs Frazier. Real close but my gut goes with Ali Frazier as slightly more impressive. By a hair.
I'm going with Duran vs Leonard. Here's why, in hindsight we knew Ali, coming off a long lay off, and not looking as sharp as he was In his return fights before Frazier, it's easy to understand why Ali loss, he just wasn't the same as he was in 1968 and before. Tyson also was subdully showing signs of regression, head movement wasn't as fast, hand and foot speed a little slower, all the issues outside the boxing ring in the media almost daily, factor in Douglass's mother dying prior to the fight, a tremendous reason for him to fight his best possible. Leonard vs Duran was out of the blue. A great big man losing to a great little man? Both in the primes of their careers, great big man usually destroys the great little man. Look at the history of boxing and their are many examples of great little men taking one step too many, ang getting beat by the good/great big man.... But in this case it didn't happen... It proves (in my mind at least) How truly special Duran was.
To me none are that impressive in that Duran beat Ray who was still not elite. Not in my mind. Douglas beat Tyson when he was not focused and slipping... Douglas then stopped in the rematch by Evander. And Frazier beat a comebacking Ali.. Had that been the real Ali of a year or two later i would say the Frazier win.
Duran's win over Leonard was unforgettable but Roberto was already an established great at Lightweight. Douglas was a 30/1 outsider who had folded against White, Tucker and Fergusson. What's more, he didn't drop a lucky punch, he pasted Tyson. Obviously. we know now how Mike had neglected himself in the build up but the whole fight was an experience in sheer disbelief. Frazier was favoured, if only narrowly, to beat Ali. Hardly a massive upset given this was Joe at his absolute best.
