What really has always struck me about Zacian is not only the sheer power creep of itself, but the sharp difference in viability between it and Zamazenta despite both having the same stat total. How they initially turned out in Gen 8 is not only a reflection of the power creep design but also how much harder it is to create a powerhouse that is defensively oriented and make it great vs making an offensively oriented powerhouse that is great. The idea behind both is clear and I like the design dichotomy: Zacian is min-maxed towards one offensive stat which it hits extremely hard with, and offensively you'd ideally like to specialize in just one side and be extremely good at it, while Zamazenta is less min-maxed and instead has very high defensive stats on both sides. This helps defensively because if your goal is to take hits, being able to endure them from both sides is a premium.
But Zacian has more tools to use its statline well. It has Swords Dance and can capitalize on its offensive prowess with great STABs and good coverage, and incredible Speed helps it as well. Meanwhile Zamazenta never really took off as well, with its moveset being less suitable for singles, and it didn't even get Body Press in its debut generation to use its boosted Defense which it can boost with Dauntless Shield+Iron Defense. They realized denying it that was too much and made it too weak and now it has the move, but that was an example of Zamazenta being the complete opposite direction of Zacian: instead of being the power creep, it was the victim of power creep and was comically underwhelming for a restricted legendary (even now it's not really *that* impressive). It's gotten better now with Body Press, but the point still stands: it is the complete opposite.
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In general when it comes to restricted legendaries though, it's interesting that the ability to be exceptional or unique means the strongest of the restricted legendaries is often made such not just by stats, but also abilities, movepools, and whatnot. Like Kyogre, Groudon, and Rayquaza for instance are not new: they are some of the oldest legendaries in the book having been around since Gen 3, and their stats aren't super min-maxed like Gen 8 Zacian was, but they still manage to be among the best and most utterly powerful thanks to their combination of abilities and movepools. Kyogre in particular has been a monster since its inception and is still one today because it has Drizzle, which summons rain, making it able to summon rain for others to take advantage of, it can also use that rain itself using its 150 Special Attack and Water STAB further boosted by rain to hit like an utter truck with Water Spout and Surf/Origin Pulse, the former of which is 150 BP at full health, and with STAB+Rain is a nuke. Groudon isn't quite as powerful but its bulk, Ground typing, and summoning sun has made it a formidable force. You get the drill.
Meanwhile some restricted legendaries haven't been so fortunate. Dialga, Palkia, and Giratina were great in their debut generation but have been power crept to hell, as they don't have any exceptional qualities being relatively average all around for restricted legendary standards, with no particularly unique tools in their boxes. Things like the Unova dragons and Solgaleo/Lunala also don't particularly stand out, and never did to begin with even in their own debut generations. I could go on but yeah.
Like yeah, we look at Zacian and Miraidon as exceptionally strong powerhouses who contribute to power creep in modern generations, but there's still an example of what constitutes a Pokemon who is exceptional even in earlier gens: while a lot of older restricted legendaries haven't aged that well, there is definitely one group that has (the Hoenn trio) who themselves have always embodied what makes an extremely good Pokemon well, extremely good, by being both exceptional at something and unique.