The hardest boss I fought was ness's nightmare from earthbound.Took me 4 tries till i finally beat him.

Crash Man from Mega Man 2. I hate him, I hate his level, the whole thing is just horrible, and then when you finally have to fight him....*shudders*

Mine was actually Psycho Mantis in Metal-gear-solid one XD.
I was just too stupid, to figure out
that you have to put your controller to port 2....
i had no internet, and none of my friends knowed this game,
so ive tried it like 200 times (over some weeks) and then gave it up.
Then i´ve got to the market one day,
and there were cheat-magazines (thats old stuffxD)
and i just read it in the market, cause there was a walktrough for MGS in therexD
i couldn´t belive how easy(afterwards!)he actually was

Mine was gannondorf in the legend of zelda ocarina of time for the n64.

So far, Yaizmat from FF12 is the hardest simply because I still haven't beat him. I've got him down to about half health, but he doesn't seem to be moving past that now. *facepalm*

My would be Alice on hard mode in no more hereos 2.
It took me about 20 tries, and after i finally won my hands were covered in blisters!?!

bazboyd : that water guy in KH2 first seymore battle in FF 10 taboo in SSBB and gary in pokemon are all tied

Any of the bosses from devil may cry 3... they make me cry like a baby

Sephiroth in Kingdom Hearts 2. He made me put the game down forever.

I say the first time you fight Wesker on Resident Evil 5 (ON PROFESSIONAL MODE). He is easy given 2 circumstances: 1. You play on any mode other than Professional. 2. If you are on Professional, you have a real person as your partner instead of the stupid AI partner. On the unlocked professional mode, for the exception of getting bitten by a dog, EVERY hit by an enemy is an instant kill. That is unless your partner can reach you in time to resuscitate you. But on Professional mode, your partner has 1 second to do so (Not a figure of speech. Literally one freaking second). So you have Wesker, who can dodge most of your bullets with his super fast movements, and you have his partner, who is spraying the area with a machine gun. You can't harm his partner because (won't tell because it is a big spoiler for those who haven't played and plan to). The only way to hit Wesker is to shoot him from behind when he is walking toward your partner, of if you shoot him a few times when he builds up his charge move, which is a window of 1.5 seconds. Problem is, he rarely does that because your dumb AI partner is shooting at him like crazy, just making him able to dodge all bullets (even from behind). And all that wouldn't be so bad if it weren't for the fact that getting hit by one bullet from Weskers partner is instant death, and she has a tendency to focus on your dumb AI partner, who doesn't seem to want to keep close enough to be resuscitated within that 1 second time limit. And what is worse is that if you are the one who is shot one freaking time, your partner is so far away, she can't reach you in time. And if you happen to be standing RIGHT NEXT TO HER, there is only a 2/5 chance she will be smart enough to resuscitate you even though she is right there. If both of you get caught in the constant machine gun spraying, you just die. Because of that stupid AI partner, I didn't get through that fight until the 43rd try (it keeps track). Most of those tries lasted less than 15 seconds because the AI is a moron. So frustrating. 

the king has not much difficulty with games but the one day i was playing the other M i saw it the hidden final boss Phantoon. this alien can phase in and out of the real world make eyeballs with flames rise from the ground and summon mini hands to do his bidding but the worst part is....... his dimensional eye laser he does that to piss me off im serious that guy is freakin EVIL x 20
i beat him after 13 tries

I would say shadow link from the legend of zelda 2. But there is an easy way too beat him. Then again I did not know the easy way when I first played so... I have to go with Shadow /Dark link.

Mine was probably that jumping jelly fish monster from Ocarina of Time. It was either inside Jabu Jabu's belly or the Water Temple...it's been awhile since I played and beat the game for the 5th time so I don't remember which exactly.

I had a really hard time beating Riku/Ansem in Kingdom Hearts. =| It took me about 10 tries to FINALLY get him because when his heath got around the half way point, he wouldn't stop charging at me and I kept having to glide everywhere. I literally cheered when I beat him.

And then of course there's Sephiroth in the second game. He's a pain. I eventually just gave up because it frustrated me too much. >.<

I have two actually. 
One was Sephiroth from Kingdom Hearts 2. I barely can get him down half way. (He takes the cake)
2: Would have to be was Krasuer from Resident Evil 4. He's still my favorite to fight though, He's bad.

Steven on Pokémon Sapphire....*shudders*.  I had no fire types or ground types so It to me like 20 tries to beat him. Then again that was the first Pokémon game I ever played.

Tabuu on intense boss battle. You have to pretty much avoid every attack. I beat him once, but then my first wii stopped working. Guess I have to do it again

Probably Sephiroth Or Riku/Ansem from Kingdom Hearts or maybe dark sora

I also find the final boss in Sonic Adventure 2 pretty difficult

Hardest thing I've ever fought...

I'd honestly have to say it was Sephiroth in kingdom hearts.

I just can't think of many that challenged me. Generally bosses are easy. It's the levels that provide a challenge.