Try and beat this :

boss came to me bitching about how some of my work was sloppy.  well i certainly dont like having my work disrespected.  so i took my keyboard and smacked him in the mouth.  fucking asshole ruined my favourite shirt getting his blood all over it in the ensuing confrontation...hes a big guy too, so a lot broken stuff around the office when we were done having it out.  (and yeah, i may have been fired, but his boss fired his ass as well, so its all good)

^he sounds like a pussy to me...nah, Darkforce is harder in my opinion.

Darkforce in PSIV, it's quite hard too. The videos make it look not that hard, don't let it deceive you....

In recent times, the Golem guy in Metroid Prime 3, in Hypermode difficulty. I think he was more annoying than anything else tho.

PSIV is one of my favorite RPGs ever, but I wouldn't say that any of the bosses are that difficult. Once you learn a few combos, and you spam them and heals/buffs, and everything pretty much just dies without much fuss. That said, some of the PSIV bosses are still harder than most of the stuff people are listing here.

As for me, I've recently been re-playing Castlevania: Portrait of Ruin on Hard Max Level 1 without using a clear data, and it's one of the hardest things I've ever done in a game. 1 hit and you're dead most of the time (against bosses) and you have VERY little manuevering space sometimes. The hardest boss so far was Death, with all his little scythes, and any of his main attacks one shot you; it was pretty frustrating. I can't wait to get to Dracula, but he'll be easier than Death alone was.

Ninja Gaiden Black in master ninja difficulty has some hard fights too. And Ninja Gaiden Sigma 2 has at least 2 Ultimate Ninja online coop boss fights that are borderline impossible and that's exactly what makes that game almost impossible to platinum, or at least the hardest platinum currently.

Killzone 2 Radec in the hardest mode. I almost commited suicide.

My hardest boss fight will absolutely have to be the Demi-Fiend/Hitoshura from Digital Devil Saga 1. The amount of preparation you need to perform to even stand a chance at surviving long enough to think about how to defeat him is insane. Almost as insane as not being able to challenge him until your second playthrough. The fight is long and relentless, but ultimately satisfying once you defeat him; the video below is just part 1 of 6.

. . . And this is what happens when you challenge him unprepared (try to ignore or mute the guy taking)

Here, this, without doubt,  is among the top 5 hardest fights in all videogame history:

The bosses in Metroid Prime 2 : Echoes on hardest difficulty (Gamecube version)