To probably stretch your definition of optional I would probably have to say Legate Lanius from Fallout:  New Vegas.  Otherwise, I know that squaresoft hides a bunch in their games, but I cannot remember which one(s) I preferred.

Ummm .... don't know that I have one, honestly. I don't know that I've fought too many.

Im stumped i cant remember lol!!

Sephiroth from the first Kingdom Hearts. I still have nightmares about that.

I enjoy the hidden quality of the Painted World in Dark Souls and the optional boss fight with Priscilla. The gameplay and battle itself isn't what's so superb about it, but more the lore behind it and the fact it exists. I felt pretty bad killing her, her backstory and the possibilities of how she came to be are interesting, and she's not a villian by the looks of it.

I agree! Although I agree more because of the whole painted world feel than the boss fight itself. It felt so much more like Demon's Souls than anywhere else in the game to me!

Don't know about favorite, but I know I hated the Ruby Weapon in FF VII. F that thing.

I usually don't like hidden/optional bosses.  They are normally over powered.  A good example is seeking out Elizabeth (Persona 3: FES).  She's insanely difficult and wasn't worth my time to beat her.  Getting to her was hard enough and trying to beat her was annoying.  

Culex from Super Mario RPG. Pain in the butt,but I sure did love that game.

Exactly! (This is what I get for leaving my browser open for so long without refreshing the page lol)

I liked Final Fantasy boss in Super Mario RPG. I don't know if he was from a specific game, actually, but it was the boss music from FFIV.

I'm surprised no one mentioned Emerald Weapon from Final Fantasy 7. That's really the only one I can think of. That dude was a beast!

I mentioned Ruby cause I hated that one way more than Emerald. I was actgually able to beat Emerald.

I'll go with Tela Vasir from Mass Effect 2 since it technically is optional the way that game's structured.

Ruby and Emerald weapon, they are stupid hard and the only thing you get for beating them is bragging rights. Ozma from FFIX is another good one. As for more recent games I would have to say Terramorphous and Vermiverous in Borderlands 2.

Not really a favorite but an annoiance was King Poo in Blue Dragon.

Sorr you got me on this one. I can't think of one that was my faviorate or optional. Ihang my head in shame.

Ruby/Emerald Weapon from Final Fantasy VII. Mainly because they were so difficult and challenging actually made play the game way longer than I had to beat them!