All Ninja Gaiden II bosses on Path of the Master Ninja mode and all lost Planet Extreme Condition bosses on Extreme mode.

I have completed both games on the hardest difficulty levels so they are not impossibly difficult for me but they are the hardest I have had to face.

All RPG bosses are easy because you can just grind and grind where as any real time action game where you can't level up your character requires pure skill. I beat Emerald Weapon only taking damage once and beat Ruby Weapon taking damage three times which killed one of my characters.

Yeah to beat the Cairo station part took me like 3 or 4 hours on legendary and I was playing coop, but my coop partner was my little bro so Is shooting wasn't that good but damn that level is hard

FFIX last boss...freaking amazing....9999 damage 12 times and it still didn't die!.

God had - This game...good luck. On level 1, they start throwing you very hard things...like Demons who pops up randomly when you kill a dude, and believe me, it's hard to kill it without using a special. The bosses are hard too.

Suikoden 1 & 2 last bosses - ...OMG. Took 6 tries to kill them both.

Tekken 3 last stage boss.

Chrono Cross optional boss - Daryl... now this dude is a pain in the ass. I didn't know the tactics to beat him.

Bosses which are hard to beat are mostly in RPG's.

Some of the random creeps in FFIV DS. Seriously there were times when i was just running along, id run into two super powerful creeps with ouchful moves and get wiped before i could even attack.
Christ FFIV is by far the hardest FF...

I actually found the bosses in FF9 to be relatively easy as long as youd taught everybody good abilities (auto-haste is truly a godsend)

Jecht in FFX (never beat him) and Metal Gear Rex in MGS (until I looked at a walkthrough, then he was okay)

I have to say I agree, the only boss i found hard was the boss you could buy things off if you beat him(near the end). The last boss was easy with counter, full life, reflect, carabuncle and phoenix ect.

Radec on KZ2 was hard to beat on elite. It took me hours to beat him.

none of the ones you mentioned are hard to beat, now if we go back to the NES era some bosses were insane

there is also a track is gt5 ptologue in the S level that you drive a F40 and have to pass all others in a single lap... that was hard ...

I honestly can't think of bosses I struggled with too hard...

I've run into some really hard an annoying spots in high difficulties with Kz2 and CoD:WaW though.