I'm definitely going to be in the minority here, but I would sing the praises of Vayne Solidor any day.
Sure, he's nowhere near as immediately dangerous as the likes of Kefka and Sephiroth. He does not swing an impossibly stupid sword around while speaking with the unfiltered sexiness of Liam O'Brien's voice like Caius. He doesn't make your skin crawl as much as Seymour does with his visible open chest and a bit of his midriff. He doesn't look as fabulous as Kuja or enjoy Mortal Kombat as much as Ultimecia does. He's also not something as awesomely silly as a warlock made out of a tree like Exdeath.
But he doesn't have to be. Vayne is his own thing, and an atypical type of antagonist at that for the franchise. He's a cold, calculating, machinating political idealist, but his overall goal is arguably noble and utilitarian. Allow man, under the helm of a strong leader, naturally, to write its own destiny without the chains of divine design. Subjugation and destruction is a necessary evil to attain a better Ivalice, Vayne would reckon.
Sure, he shares no real personal connection with anyone in your party, and nor does he have that constant presence that other villains enjoy, but for the type of story posited by FFXII, Vayne works well for it. He doesn't break any new ground, and he's a bit cookie cutter for an antagonist of his tropes, but neither is he a rubbish villain with nothing worth talking about, as detractors often say about him. Do we have to hate a villain on a sort of personal level in relation to the main character? Does a villain have to be unambiguously evil to be good? Of course not. In many ways, you can argue that Vayne can be subtly more dangerous than others precisely because he isn't unambiguously evil, and because he has the honeyed words and oratory skills to soothe the burning anger and frustration of subjugated citizens (see the cutscene where he makes his maiden speech in Rabanastre as Consul), so he can easily bend and manipulate people to his cause.
I feel that Gaius van Baelsar in FFXIV shares some similarities, but unfortunately, the narrative is nowhere near strong enough to convey the character across well. At best, he appears in the final dungeon to exposit EVERYTHING at you in a cutscene that new players tend to skip, to avoid being left behind by a party dead set on rushing through it. But ultimately, Vayne is more dangerous than the Legatus, because for one, he wields absolute power unlike the Roman general, and because while Gaius is played like a fiddle by one immortal entity, Vayne has a close, working relationship with an immortal demigod.
Vayne, along with Balthier and Cid, Gabranth and Basch, and Fran and her people make up the most interesting parts of the game for me. The exhibition of different family values, and what characters will endure or neglect for their own clan is fascinating. People claim XII doesn't resonate emotionally because of farther apart scenes and a political slant, but what is more personal and emotional than a woman saving one of her kin and imploring them to stay with the family they have, to escape the ostracization that wanderlust created?
Or a son who is forced to commit patricide on a man who neglected him his entire life in the name of research that perverted his mind?
A pair of siblings torn apart by the dark chasm of war and standing opposite each other, both wishing to rectify their own failures by flying a banner foreign to them.
FFXII had great writing and a great scenario. Vayne is awesome, too.