FireX

golem twins aren't too hard, you just need to have one person with a vest/mail that absorbs one element, then another person who has magic/techs of that element... use that always (you can mix and match, too, like give one person a yellow vest, another person a blue vest, and the last a red vest) and each counter attack will heal one person.

i think the stage 4 boss of Mars Matrix is pretty hard. then again, I'm probably missing something obvious about his patterns... Edit: I also went through a couple hard boss fights in Dragon Quest 6 recently... 1) The series of battles vs Duran (don't wanna give too much away, it's a cool spoiler in the story), even though he restores you to full HP/MP and revives any dead chars before the battle starts he's insanely hard. 2) Akbar, he's hard too. There's one spell that can work on him to make the fight a lot easier, but even with that spell he'll kick your ass.

the doom games are fun, but the cyberdemon and spiderdemon are too easy to kill... just use a BFG and circle-strafe them.

RPGs: Dark Falz in Phantasy Star 2, the last Dharma Shrine boss in Dragon Quest 7 (please, don't make me laugh by mentioning FF bosses. the only way Square can make them "hard" is to give them instant-kill moves where 1-2 overlooked abilities will make them a complete wuss). Shooters: Gradius 3's Stage 3 boss. It's not impossible, but I find he's damn hard, harder than any other boss in the game. Fighters: Nightmare, on the hardest difficulty, is a pain. But Cervantes in Soul Edge was more of a pain. Well, anyway, there's also the aforementioned SNK bosses.

The link in the main post is no longer on the site, since you switched to php. Just thought I'd give a heads-up.