Worst: Yiazmat in Final Fantasy XII.
Ridiculously annoying and long fight. It wasn’t that it was even particularly difficult once you figure out how to beat him, just extremely tedious.
He has 50,000,000 hit points. Has unstoppable instant death attacks. Can hit your entire party with a wind attack that does 6000+ hit points of damage even with air resistant armor (i.e. the light armor “Windbreaker”) quickly equipped (and does something like 9999 damage to everyone if you don’t have that armor).
Each melee attack does something along the lines of 3000+ each and he chains them.
And he can also employ a stone breath attack that gives you 10 seconds to prevent a character from turning to stone.
I heard one person defeated him in 4 hours. Kudos to them. It took me somewhere between 6 and 8. Yes, that long of a boss fight. And unfortunately, you just can’t set your gambits and walk away. You have to manually do some things. Fortunately, you can run away from the fight and save (though, when you run he really breaks out the instant kill ranged attack, so you will probably lose your primary three right there, but you should be able to have at least one of your six still alive by the time you get to the save crystal). The only disadvantage to saving is that he restores all his buffs, but unlike when you run from Omega to heal up a bit, he doesn’t start healing himself (the only redeeming thing about this boss fight).
No fight in any game should take longer than an hour using even a halfway decent strategy. There’s nothing wrong with making the strategy hard to figure out, but once it is, the fight should be fairly quick.
And the rewards for defeating him are lame. So you get the best sword in the game (from a speed, power, and chaining perspective–the Zodiac spear is the most powerful, but it’s slow and rarely chains). Big deal. You’re high enough level at this point that you could wipe the floor with the final storyline boss using the weakest weapons in the game.
You also get a little bit of more sidequest story filled in, but the whole experience is anticlimactic after this fight.
Bad design all around.