Out of the given options I'd say it is a tie between Hojo and Kefka, although I do have to admit that I don't even know who Gerrith Barrington even is.
I never played the MMO FFs and beyond Type-0, Dissidia and Theathrym none of the Spin-Offs.
When it comes to other FF villains though many of the early ones where megalomaniacs who wanted to either rule or completely destroy entire worlds or even universes. Zemus/Zeromus from IV is in good company with Kefka and Hojo, as is Exdeath to a certain degree.
The Cloud of Darkness from III is a special case as its more of a force of nature rather than being actually "evil" per the usual definition.
Ultimecia with her time compression is a runner up too however, also since in FF VIII itself it is insinuated that only she herself can truly exist in that time compressed "world".
Not to mention that when you reach her time period one of the first things you get to see is a bunch of corpses scattered around the base of her castle.
FF IX and Kuja are an interesting case as while many of the things Kuja does and causes are truly heinous "evil" and on par with the others he's among one of the few "main" FF villains who has some sympathetic angles to him and who (at the end) is capable of at least slight remorse, that doesn't scream "most evil".
In IX there actually isn't exactly a 100% evil antagonist around despite several wars, outright destruction of whole countries and genocidal actions instigated by both Kuja and Garlant.
In X, Jecht is actually more of a tragic anti-villain and Seymour, while callous and nihilistic, can also count as a broken soul rather than being truly evil. Yu-Yevon could arguably count but by the time of the actual game he's more like an instinct-driven plague rather than an intentional malicious evil.
Vayne Solidor from XII may arguably the very first full grey villain of the FF franchise as he's more of the noble demon type and doesn't do "evil" because it's fun or just for his own sake, but to free humanity from the history-controlling grasp of a council of jerk-ass gods and while he does take power he intends to pass it down to his peace-loving and more "good" younger brother once he has spilled all blood necessary to unite the world under one banner.
Vayne also fully acknowledges and knows that many of the things he does are morally inexcusable and while he regrets having to do them he is ruthless and ambitious enough to go through with them for the sake of the higher cause.
XIII on the other hand is murky mainly because the main villains aren't human but some god-made sentient machines. Both Barthandelus and Orphanus come over more like the early FF villains who are "evil" for the sake of it but have actually a completely different mindset and do not even see humans or human-esque beings as equals in the lifecycle. Cid Raines and Yaag Rosch are the anti-villains and noble demon types respectively and while both work for the antagonists they're not really "evil". Jihl Nabaat is a different story and arguably the most "evil" human villain of XIII, but not really comparable to contesters like Mateus, Zemus/Zeromus, Exdeath, Kefka or Professor Hojo.
Now XV, well, haven't really played that one yet beyond a few hours in the first chapter, but what I've read and seen on the net about Ardyn Izunia seems to catapult him right into top contester territory for most evil despite him apparently also having a tragic and sad backstory.