Those kinds of items crafted in ages long past by gods or the like, with power beyond the comprehension of current item crafters, yet conveniently built with Hermetic guidelines because that's the easiest way to rationalize effects... Err, that last bit isn't actually a requirement.
I imagine they'd probably have the unique ability to give attunement bonuses despite not being talismans, in order to support their "element," as well as a number of individually powerful effects that may or may not follow all Hermetic limits. I don't know if I'd go for the traditional eight elements, or rather the ten Hermetic forms, or maybe some other combination of things.
I was thinking they'd take the form of orbs, simply for the sake of being thematic, but constructed of materials that give appropriate attunement effects. Maybe put a curse on them that lets the orb effectively control anybody who can't willpower through it. Or maybe just CrMe on all of them to create and enforce incredibly strong feelings of wrath, or maybe item-dependent feelings that nonetheless cause problems. Turns a kid who accidentally picked it up into a fair threat, a magus of any age into a monster, and a master magus (who can likely control it) into a titan.
So... Aside from that... What kinds of effects would be on these sorts of items? What kinds of powers would be invested in such an item? How strong would they have to be in all their facets to earn recognition as the most powerful items, not replicable by current elemental masters? Given what a Hermetic master can accomplish with a focus, fully developed magic (TeFo 129 thanks to focus and Puissance + 24 MT (Affinity + Puissant) + 3 Inventive Genius + 5 Int + 10 Aura + 13 Familiar + 6 Apprentice + 24 (best possible Shape/Material bonus) = 202, plus as much as 12 if experiments are done to 112 with a high Leadership and several other Magic Theory masters willing to put time into the success of the project, so 302 or 314ish... Scary!) it's hard to imagine meaningfully outdoing what that Lab Total does with even Lesser Enchanted items, much less a full-on invested effect with a year or so put into it, as beyond that point it's sort of just a bigger version of the same effect with more Penetration rather than really reaching into the territory of un-doable effects. The exception would be if it broke important Hermetic limits, but then it's just a flashy version of some other integration-ripe non-Hermetic item rather than the strongest item to have graced the Order with its appearance.
Now, of course, perhaps this works simply with a change of standards. Even without actually outdoing those highest-tier-possible effects, the elemental items could match them at least, be invested with multiple of those uber-level effects each, and of course there are multiple items. Even if the orbs don't theoretically surpass what Hermetic magic is capable of at its greatest aside from pushing or breaking a lesser limit or two, an item with so much power as to require the Order to bring together several of its mightiest Archimagi and spend literally several queens of vis on just to reproduce would certainly be an ultimate item by most standards.
My last idea, since the option of abusing non-Hermetic principles exists, would be to make the items base their power off of their weilder. So a random peasant becomes threatening to soldiers and newly-gauntleted magi, but a magus with a few years under his belt could quench the situation; but if that magus of a few years, specializing in the orb's element, got ahold of it, it would provide a much bigger boost, making him a concern for magi far beyond him in years; and if a major specialist in the element, at the peak of his magical ability, grabbed it and could control it, he might face the Archangels single-handedly. I'm not really sure how you'd regulate the growth curve of something like this, though, so it's the least reliable option I can see. Better to just make the item totally brokenly powerful for everyone.
What do you guys think? What kinds of elements should be used for these items? Might they contain Parmulae-esque enchantments, boosting ME by 30 (even if you started from none at all)? What kinds of effects should such items have, and how powerful should they be? Any other commentary on what might bring a magic item to the status of being considered "the ultimate" and superior to any modern peer?