For me it's a tie between wily capsule in megaman 7 and the dark demon in demon's crest. Two insanely hard bosses that will make you angry in seconds in my opinion. What are yours?

One of the hardest bosses I have ever faced was Kesseler from Infamous, it was right after getting the game for free because of sonys entire hacking ordeal but I ended up beating the game in 3 weeks and one week of that was battling him at least 20 times a day before I would get frustrated and quit.

the ole rage quit happens to all of us, that kesseler guy doesn't look too hard though. But i guess you could say that about my bosses and then you fight them and get smashed.

UFC : I just recently beat Megaman 7 for the first time. That means I have beat 1-10 now. That Wily fight in 7 is by far the hardest of all the original series games. I know it was my first time fighting him but wow...

Some other things that come to mind are:

Doppelganger from Castlevania 3
Slash from Turtles IV: Turtles in Time
Joker from Batman (NES)
And of course Iron Mike from Mike Tyson's Punch Out who I have never beat...

My hardest boss fight was the last stage in New Super Mario bros Wii. The last boss was Browser and he was easy to beat the first time just by simply waiting for him to jump up one could easy go underneath him and press the red button to drop the bridge that we are fighting on so that way he can land in lava well I did that part and then the wizard guy who flies around on a broom came and revived him and made Browser huge and he started smashing the level and your only choice is to out run him and let the castle cave in on him.

Currently the only one that comes to my mind is "Gill" from "Street Fighter 3". (The Blue Skinned Guy who keeps Regenerating his Health with "Resurrection".)

The "Grim Reaper" in "Castlevania 3". (The sucker changes form and "offs" you before you can "run a bead" on his pattern.)

I am certain there Should be one from the "Ninja Gaiden" Series but those games are notorious for being difficult. (You don't think about the difficulty. That's the point.)
.................
Ninja Gaiden Black for X-Box or Playstation 2.
I'll go ahead an mention the Tank battle at the train station Followed by 3 Tanks and a platoon on the next stage.
I think I took a week long break after I saw the three Tanks.
The first Tank takes you a day to figure out how to get around. (at least that's what it felt like.) Then you progress to the next stage and there are Three Tanks and a Platoon???
I am certain there are harder Bosses in that game but the most frustration ones are the tanks. (It's a good frustration too. Because after you destroy them? You feel like a god. ...Then you have to fight a mega demon that is a god a couple levels after. It never ends with the Ninja Gaiden games.)

"Anubis" from "Zone of the Enders 2". (After getting to the end of that game? You might want to take a break and pop in strategy guides on-line or check for Glitches. Many games get like ten levels easier by just taking a ten minute break every 30 minutes to an hour. (Common glitch. I think modern games should embrace multiple disks if you can not play a game for longer than an hour before it starts glitching out. Seems like a silly problem to have for all of the cash we have to fork over to play them.)
Any-hoot. .. Anubis is a bit of a Handful.

A big example of the "10 minute break glitch" is "Homefront" the F.P.S. game.
The entire thing. About Every 30-45 minutes you have to take a break or the Enemy A.I. just swarms you and spawn camps you till it is a fruitless effort. Turn the game off and walk away for ten minutes and everything is back to normal.

Many modern games are only difficult if you do not know how to use everything in your arsenal.
Devil May Cry 3.
The First "Virgil" Fight was hard till I learned how to BUY power-ups like health stars, devil trigger stars, and Holy water.
Bayonetta.
The First "Cardinal Virtue" Fight was hard till I learned how to MAKE Lollipops.
and so forth and so fourth.

There are many difficult Bosses but you have to find them now. You have to wait a year after a game is released before someone musters the balls and the humility to say "hey, that fight was hard".

I probably should list more classic Boss fights from the past but I am too tired to remember right now.
Good Luck.

Good Luck To All.
I'm done. Stick a fork in me.
Peace.

nice answers guys, i never thought doppleganger and the grim reaper were that hard from castlevania 3, I just beat that game and the level 7 3 bosses in 1 annoyed me more.

To be honest I all ways found the first boss on batman returns for the snes hard mainly due to the fact I keep forgetting how to beat him

Dracula on castlevania 1 is a real ass, very annoying and tough to beat   

The hardest boss fight I would 2 say is omega weapon on final fantasy 8... he is the hardest boss in the game..and I hate when he does his supid Terra brake cuz it kill u n one hit n u really have 2 b quick about it when have fight him

My hardest boss fight ever was the boss at the end of the first Bowser Jr. level in Super Mario Galaxy. I could not beat it even with a walkthrough so I ragequit and sold the game to the store I bought it at.