Most Powerful Boss EVER

Savior from DMC4? I mean he can like destroy the world.

G-Man, you might never physically fight him but he runs things.

i found volvega and the witch sisters really hard on ocarina of time, but that was when i was around 13

No one can talk about powerful bosses without mentioning Mike Tyson from Punch Out. I still haven't beaten that bastard.

the last boss from Shinobi hands down.

Mike Tyson was a tough boss at the end of Mike Tyson's punch out.

Poop monster from Conker's Bad Fur Day. Hands down.

   The Hand from the original SSB.

Another difficult and powerful boss that comes to mind is Culex. He was an optional boss in Super Mario RPG. The fight was pretty cool because the battle music from Final Fantasy IV played.

Ultima Weapon from FFVII

"G-Man, you might never physically fight him but he runs things.
"

The King of All Cosmos. He was both a boss in terms of commanding and a boss in terms of he was the last thing you could roll up, and he was bigger than the sun.

Mike Tyson in Punch Out! comes to mind.  I also recall the T-Rex at the end of Turok (this year's version) being maddeningly difficult to beat.
Still, my favorite and most hated enemy had to be Dracula at the end of the DS's Castlevania: Portrait of Ruin.  You have to get the special ending to even fight the ol' bugger, and when you do, he's flanked by the Grim Reaper and the two of em can take your health out in two or three hits.  And just when you muster enough strength to beat the two jerks, Dracula absorbs the Grim Reaper and you have to fight a horrible demon Dracula.  I'd be lying if I said I beat this boss after 10 tries.  Heck, I'd probably be lying if I said I beat him after 20 tries.