The most difficult boss you've RECENTLY defeated.

[Mirroga]

Egh...that's a tough one.
It's a toss-up between Mom's Heart, War or Gurdy*.

*(who isn't that tough once you figure out his gimmick. Gurdy Jr. is actually quite a bit tougher since he's much, er, faster)

I always gave the honors of hardest bosses in BoI to Mask of Infamy and The Bloat.

 

[Nickthequick]

Bonetail from Paper Mario: The Thousand Year Door.
The actual battle wasn't really that hard, just getting there in one piece was long and arduous.

 

[Battle Pope]

Crawmerax from Borderlands, again.

Got a fantastic Atlas machine gun. Hooray for useful loot!

 

[Soxafloppin]

I'm currently playing through the Ratchet and Clank trilogy, Chairman Drek from the first game took many attempts, the Protopet in the second was a joke though.

 

[Slash Dementia]

Fighting Letho for the first time in The Witcher 2. Even on Normal that is the most fucking infuriating bullshit I've ever come across. It's literally just spam traps and hope you do enough damage and then run away and repeat. I must have died over a dozen times and the fight against old tentacle face before wasn't much better either.

Captcha: get your goat - I'm sorry, what?!

That fight was something, and I had a really hard time with it. Letho did way too much damage--it didn't even feel like Normal difficulty when fighting him.

My fight was Ornstein and Smough. The first time I went through the fog, Ornstein charged at me, I fell, Smough had already caught up, he smashed me with his hammer. I tried running away to build up a strategy, but they kept going at me. It was just a hard battle, though I got it on my second try.