Bosses you REALLY want dead are the irritating ones, the people you can't stand. So, I choose Seymour Guado, from FFX.
Not that I need to explain why, but let's review anyway. For laughs. Seymour is a pompous git who's always full of himself, born rich and royal, with his own personal Aeon at his command. He's a spoiled evil jackass who gives decent villainy a bad name.
Seymour's stint in FFX is as follow...
"Mwa ha haaa... I'm the guy in power and there's nothing you can do to stop me!"
*Dies*
"Mwa ha haaa! I'm undead now and there's nothing you can do to stop me!"
*Dies!*
"Mwa ha ha haaa! I've been stalking you on my undead chariot of doom and now you're finished at last!
*Dies Again!*
"Mwa ha ha ha haaa! I'm inside of Sin for some reason and I'm totally going to take it over from the inside, even though I have no idea how to do that and almost no chance of success!"
*Dies...Embarassingly*
The problem with Seymour was that he was an idiot. He had everything he could want handed to him, but that wasn't enough...so he murdered his father to gain even more power. But THAT wasn't enough for him, so he also wanted to become Sin for some reason. But then, THAAAT problem wouldn't be enough for him, so he'd probably try to be something else, like the friggin' parasite that started this mess in the first place!
Also, his final battle was totally a joke, and I found it quite satisfying to easy-street him to death. The man has color-coded elemental panels that determine the type of spell he's using, and they all start out on the same one, to be changed by anyone who happens to strike 'em. When I fought this guy, it was all fire, so I went "Ifrit, go teach this guy some manners!", and it worked like a charm. Seymour couldn't kill the damn Aeon. Not until he FINALLY shifted the element panels to ice. Then, I just let Shiva in and bam, done. So sad...