Fist fight? Well we've never really seen how good Thanos is H2H...so I'd lean more toward classic Juggernaut. If it were wrestling, then Thanos takes it.
Juggernaut. It's not really a question even. I mean, if this is purely a fist fight, then he is the very last one to be standing, no matter how long it takes. Both Superman and Thanos will EVENTUALLY become fatigued and wear themselves out, setting themselves up for the KO. Juggernaut, meanwhile, will still be fighting as fresh and strong as before. He's the most invulnerable and has the most stamina/endurance. So Classic Juggernaut will take this after a few hundred years of fighting.
Thanos and Juggernaut are kind of the same animal The have Godlike Durability Juggernaut has just a little bit more but Thanos can't be harmed Physically
If Superman has the power to hurt Juggs then he wins this.
And if not, then he stalemates Juggs. This is because Juggs would never be able to hit Superman (barring PIS) and the fact that Superman would constantly hit Juggs far away indefinitely.
Now Thanos comes in a distant third. He doesn't have the physically blunt durability to withstand punches from Superman and he doesn't have the required strength to hurt Superman or Juggs (definitely) significantly enough. Even Captain America made Thano's head snap back from a punch (it didn't hurt though).
Id like to see you go to herochat and post this proSuperman talk. Thanos could take punches from Superman all day and has durability that makes Supes' pale in comparsion.