As far as strength is concerned between all three of these guys it looks like this:
1) Mindless Hulk, Maestro Hulk, War Hulk
2) Hercules
3) Merged Hulk (or Professor Hulk)
4) Juggernaut
5) Savage Hulk
6) Calm Hulk (Green) who is well below both Hercules, and The Juggernaut in strength feats.
7) Mr. Fixit (Grey Hulk)
Note* All of these Hulk incarnations have the potential to be stronger than both Hercules, and the juggernaut depending on their excitement levels.
I know for a fact that the Mindless Hulk would crush Hercules in time as he manhandled him (The Incredible Hulk # 316) even with the help of Wonderman, Iron Man, Namor, and Doc. Samson, who are all near Juggernaut in Strength... had the fight continued it would no doubt be Hercules as his last opponent, and without the help of the rest of the Avengers Hercules was destined to go down.
As far as Juggernaut is concerned, he has a near impervious forcefield, but apparently Onslaughts body armor was given a similar rating, and we know what the Hulk did to his physical form, so Juggernaut loses as well.
If this was a fight between just the Juggernaut, and Hercules neither of these guys tire but one of them will begin to feel pain, and that would be Hercules.
Even though Hercules outclasses Juggernaut in sheer strength, this fight will go to the one with the better body armor, even Hercules would begin to feel Juggernauts fist, but I've seen Juggernaut sustain a direct hit from Thors hammer without being hurt or even fazed.
Unless someone can come up with substantial proof that Hercules can punch harder than Mjolnir can hit, I don't see Hercules even fazing Juggernaut, and unless Cain allows him to do so, this would be a classic case of plot induced stupidity, Hercules will fall just as he would from the beating that he would receieve from a mindless Hulk or even an amped up Savage or Maestro Hulk.
In my opinion juggernaut has all the tools to be a Hercules killer, in the same way that Spiderman's nemesis Venom is to him. The only way that hercules could beat a classic Juggernaut would be to fight him with agility and craftiness, but toe to toe, blow for blow, Cain will emerge the victor. Note* Trion Juggernaut went toe to toe with a very healthy Thor, and put him in a bearhug nearly breaking his back, had Thor not found a way out of this he would have been broken, i have never heard or read anywhere in marvel stating that Hercules was stronger than Thor, but I have read that Thor slightly edges hercules out in the Strenth division. I'll let you guy weigh the evidence, and if you are not in denial of the facts, then you will see what I do.
And by the way I'm a Pitt fan I'm not partial to any of the guys listed above, I'm just pointing out what was written, by the people who created, these characters.