According to Wilikpedia was the Mutant Juggernaut who was as strong as Colossus and then got the Cyttorak Gem. Well if Cain Marko had regular strength before the Change. Then a Mutant with Superhuman class 100 strength would be extremely stronger then the original Juggernaut. "In the Ultimate Marvel continuity, Juggernaut was apparently a mutant drafted into the Weapon X Program. He did not appear to be the indestructible, mystically empowered villain from Earth 616, but rather a mutant with superhuman strength comparable to Colossus. His real name has not yet been revealed, and he was liberated from servitude when the Weapon X Program was terminated by Nick Fury and S.H.I.E.L.D. Recently, in the Ultimate X-Men 2005 annual, Juggernaut was bonded with a gem stolen by Rogue and Gambit called the Cyttorak Gem. Though its full repercussions are unknown, Juggernaut did describe himself as feeling "better than ever".
Just because a reguler guy in 616 gets the stone and becomes class 100 at least doesn't mean that a Class 100 mutant gets the stone and becomes waaaayyyyy stronger. The Gem probably want amp up his strength as much as the 616 did I mean this is Ultimate the same Morons that ripped off Etrigan the Demon to make the Green Goblin (just look at him and his powers jeez) and made Thor a cult leader. And regardng whether he beats anyone on the list, no one and why is Sentry Higher on the list then superman?
If the gem in 616 and the Ultimates universe turn out to be about the same then well Ultimate Juggernaut will just become classic version then.
Once he had the Gem all it did was give Cain unlimited durability and strength hard to improve that simply because you start off stronger.
Anyway the only real people with powers the Gem would help out are people like Xavier or cyclops because their powers would be amplified and so on.
Also people like Strange or Fate who have knowledge about magic and therefore can utilize the Gem better.