Hardest first boss ever?

[Swordkirby]

In my opinion, the wheel unversed from kingdom hearts birth by sleep. It is simple and somewhat difficult up until the second half, It charges that are insanely fast (faster than your dash dodge!), does a *beep* load of damage (over half of my health bar!), it was not fun.

[Bikdark]

Twilight Thorn was a huge bitch first time around in KH2

[Beed28]

FireMan.EXE in Mega Man Network Transmission. He was the only boss I eventually managed to defeat without resorting to an Action Replay. The entire game was just downright insane.

[Ceridran]

The Mega-Censor from Psychonauts.

.. er, no, maybe I got this wrong: it's just the hardest first boss I've faced outside of Terraria and Starbound

[BiscuitSlash]

FireMan.EXE in Mega Man Network Transmission. He was the only boss I eventually managed to defeat without resorting to an Action Replay. The entire game was just downright insane.

Darnit, was going to say this. GutsMan.EXE from the same game should probably be hardest second boss ever though.

I remember Endol from Bomberman Hero being pretty damn difficult at first. The electric things that move around him that you have to either run with or jump over will suddenly change path, which can easily catch you out.

And from Spiderman 2: Enter Electro for the PS1, the first boss was Shocker, who you fight in a burning warehouse. His attacks where pretty hard to avoid and he could normally use them on you even when you're trying to hide or something. If you don't do the fight fast enough, the warehouse burns up and you fail. It turns out that you can easily deal huge damage to him if you use some of those huge handing crates to your advantage by pulling them and making them crash onto him from behind, but if you don't know that, it's a really difficult fight.

[TheDoc]

My first Megaman of all time was MMBN3 (which is a great game. You need to get it) I was about 6-7 years old when I got to Beastman.Exe. I almost ripped the hair out of my head, but after 30 or so tries I got it. FINALLY I WAS DONE WITH THAT DICK OF A BOSS, RIGHT? HAHAHA F*** NO. I had the privilege of fighting Beastman V3 later in the game. I did eventually beat him AGAIN but that was a scarring most moment in my young gaming career.

[Jman]

1:50 is where the madness begins...

[doofusroad]

For me, it'd be Poppy Bro from Kirby's Avalanche. It took me FOREVER to beat him. (I dunno if this counts since technically there are the 3 tutorial stages before him)

Other Candidates: The enemy from Yar's Revenge, Larry Koopa (SMB3), Hello Kitty (IWBTB), and Whispy Woods (KDL Extra Game)