Well, lets turn this around and see what will happen.
I know that is a harder list to compile but it would be cool if some if you would take yourself time and write something down.
Just think about frustrating and disappointing battles. I think there are some.
Starting with 10, going to 1, wich is the highest position.
10. Seth from SF4.
He just seems very random with his movelist wich consists nearly only of moves from other characters and his strange looks. Not a cool villian at all. I also hate to fight him online.
9. Sin from FFX
All this talk about him for 30 hours or so and then yeah... it's flying whale with a city on top of his head. Stopping abusing drugs so hard, Square.
8. Mundus from DMC 1
I really loved the whole game back in the days and every Boss was extremely awesome and satisfying to fight against but Mundus is just kinda weird. He is the Devil and looks like a giant Statue, plus the fights against him are not that entertaing or funny as many other fights in the game. Wich was lame for the last boss.
7. Abyssal Leviathan from Skyward Sword
He has a huge build up time, while you run around on the ship and he crushes it into pieces and when i finally saw him... he looked absolute laughable. Like some Sesamestreet reject. I still can't believe how ridicolous and stupid this boss looks. And the fight overall is also pretty lame. Except for the 2nd Phase of the fight. Wich was ok. But not more than that.
6. The Hades Witch from Terranigma
You know, there are some bosses that are just destined to kick your ass and to kick it very hard. The Hades Witch is one of them. She appears relatively early in the game and gets you by surpirse. She has tons of HP, mean unpredictable attacks and deals massiv damage. If you don't start to grind hard at this point, it is over. Thinking back to this fight it always got on my nerves. I mean she is waaay stronger than the last boss in the game.
5. Salabog (Swamp Snake) in Secret of Evermore
Another one of those bosses that come early in the game and are worse than the final boss. I remember getting crushed by this bastard everytime i played this game. He is ridicolously hard and i don't know if he looks awesome or retarded.
4. Cort (Final Boss) from Legend of Legaia
Hm. This boss is probably the weakest Endboss i have ever fought in a videogame. I was underleveled and had major problems getting through the last stage and beating the mid-bosses, so i thought the Final Boss would be impossible to beat. I didn't even try to defeat him, i was just randomly spamming combos and spells and before i knew it, i had him defeated.
3. Malus (16th Colossi) from Shadow of the Colossus
I don't know. I really didn't like that fight to much. There are many funnier and more entertaining boss battles in that game. And for a game that is basically build around boss battles solely, the Final Boss is not that special. Don't get me wrong. I actually love this boss fight. There is a lot to it. It's just probably the best game ever. And this fight wasn't the climax of the game for me. But it had to be it.
2. Aumann Anubis from Zone of the Enders 2
Another final boss. He looks awesome, the music is epic, the battle mechanics are great as always in ZoE games, he is your mortal enemy and rival for 2 games and you never get your hands on him. But wait. HE ANNIHILATES YOU! AGAIN! AND AGAIN ! AND AGAIN! AND AGAIN! AND AGAIN! Seriously. This is the second hardest boss i have ever encountered (the hardest is on position 1). I must have tried this fucker 20-30 in a row and couldn't beat him. He takes you out with a few hits, he can heal himself and he moves with the speed of light. It was overall depressing to fight this guy.
1. Mysterious Figure in Kingdom Hearts: Birth By Sleep
This. Boss. Is. Impossible. Simple as it is. I had the best equipment, the most solid tactic there is against him (wich doesn't mean much) and Level 99. But this Boss doesn't care. He can still kill you with 1 attack. You heard me. Sometimes the fight starts, he charges at you before you can even react because he is so incredible fast and simply takes you out . With 1 fucking attack. The fight literally lasts sometimes only for a few seconds. And you spend 100+ to build your character. He even steals your attacks so that you can't even do anything but dodge till you get them back. I never been able to beat him. I probably tried him 30-40 times, before i gave up.