Depends what you mean by bigger. Hagler/Leonard was the biggest money spinner and had the most widespread interest and hype of those fights I believe.
Leonard-Hagler probably generated the most mainstream hype. Duran-Leonard 1 was the "greatest" fight of them, in terms of action. Leonard-Hearns 2 reportedly actually generated the biggest paydays for the fighters.
Hearns/Leonard was the best fight of them all since both guys were young and could fight different styles. if you are not mentioning Hearns/Hagler which beats all the fab 4 fights. Hagler/Leonard was the most anticipated.
Leornard/Hearns was HUGE: in an era when there were only two titles we had two totally legit champions.
From what I remember srl/hearns was by far the biggest and most anticipated. People were curious what would happen with duran and hagler but they DEMANDED srl v Tommy
Leonard-Hearns was 6 years in the making and the most anticipated fight. There's even rumors that went around that the ever retiring leonard was biding his and waiting for hagler to be past his prime before he came out of retirement for the nth time and fight a slow hagler. He refused a rematch after the fight because he said his main purpose for unretiring was to beat hagler. Only to unretire and retire agains several times later. I lost count. lol
I would say Hagler Leonard was probably the truly first Superfight, in the way we understand it today.