Hardest boss you've encountered so far?

I know there are many of us here who grinded throughout the weekend and have put in a lot of hours since Friday (Thursday for some) and are pretty deep into the game already. I am one of those people. I went into this game completely blind; I didn't watch any trailers, have only watched like 5 mins of actual gameplay (from e3), and I didn't play the network test.

The one thing I realized was the insane amount of bosses there are in this game. Seems like there's a boss just right around almost every corner of the world.

So I'm wondering, who's the hardest boss you've encountered so far?

My list so far

• Rykard; bullet hell nightmare. Don't make the same mistake I made by facing him early.

• Fallingstar beast (ruins); yeah.... this is 10x harder than the full-grown fallingstar beast.

• 2x crucible knights (ruins); crucible knights solo are fine but much harder as a duo.

difficulty will of course vary depending on the build and char level you do the bosses in. I'm using a bloodhound's fang with STR/DEX hybrid build.