I think I have managed to nail down a definition for a Juggernaut, and by that definition the following of the 55 original TRO3025 mechs would be juggernauts:
Hunchback, Victor, BattleMaster, Stalker, Cyclops and Atlas. Does this sound right, or has a missed one?
Basically, the juggernaught has chosen Firepower and Armour from the old adage of "Guns, Armour, Speed - Pick 2" line of thinking.
I wouldn't put the Victor in there given that it's 4/6, jumps and has mediocre armour on it's best variant and criminally thin on the worst.
The Battlemaster is a maybe, 4/6 is still quick for an 85 ton 3025 design, but the armour is solid and the weapons are good, so it gets included.
Stalker, maybe, the LRM's and LLas' aren't short range in 3025, but it's reasonably armoured and certainly meets the plodding criteria.
Cyclops has paper armour for it's weight, and is a 90 tonner moving 4/6, I would contest the book saying it's a juggernaught.
The Urbanmech is an excellent light mech example, and despite the long range nature of it's weapons, the Awesome would also be included. The Orion, being 3/4 of an Atlas, could also apply, but I would say it's closer to a trooper than anything else.
You've got to remember, this is 3025, Assaults go 3/5, heavies are 4/6, mediums are 5/8 and lights are 6/9.