Easiest: the captain waddle dee in Kirby super star... it's not a mini boss, It's an actual boss. I know because it's in the arena of champions or whatever where you fight all the bosses in a row. it takes 1 hit from any power in the game... even a slide kick.
Hardest: I'm admittedly bad at the FF series, but I couldn't beat Kraken in FF3. I died several times. :'(
Hardest: well, other than mike tyson in punchout (maybe too old for you guys?) Id say the final bossin .hack quarantine... i tried for over a year to beat it... and after.... at least 150 attempts, can still not beat him. I basically gave up on the game.
Ironically, the boss isn't the final... Cubia battle at lvl 90... apparently there was more to the game after the battle, but it was like 2-3 hours for each attempt, so you can see why giving up would be more appealing.
Easiest: The Elder Scrolls IV - Oblivion on XBOX 360, it's Maheranus Dagon, a huge demon but he is easy. I didn't even attack him. It's a battle between him and an Emperor but still a boss. ^^
Hardest: Jack of Blades in Dragon Form on Fable - The Lost Chapters on XBOX. I had the maximum health and he almost 2 hit me :O I had to heal almost all the time. His fire is extremly strong. He summons strong creatures to that disturbs you. :P
Final fantasy: easiest: Gerogero and abadon from ff8. one phoenix down and they die ^^ Hardest: i'd have to say ruby/emerald weapon from ff7, that is until you source farm.
I just blasted him with GFs. He got pwned. (usign emulators I'm on disc 2... Not very good at it though.)