Hardest: Yiazmat from FFXII - The thing had about 50 million HP (no joke), and if you manage to get it down to 10% health, when you leave the battlefield to heal yourselves or buy more equipment, IT heals itself slightly, and its' level increases by one. Beating it in 4 hrs is considered to be fast.
Easiest: Ozzie from Chrono Trigger (just hit the switches)
Hardest: for me, dont laugh, was the 2nd boss of Zelda OoT (i was only 8 at the time). I had the game for like 3 years and i still couldnt beat him. But then I got the game of the VC and he was easy.
Easyest: That would be the last Bowser on Mario Galaxy (Is it just me or have games gotten easier? Or is it just the Wii?)
Easiest: Bowser from Super Mario Sunshine. Hardest: Between the multiple fights with the Kingdom Hearts 2 last boss and some of the collosus' in Shadow of Collosus.