Battle of Ilum
Kaon Under Siege (normal bosses)
Kaon Under Siege (bonus boss)
All just opinion of course. Different people will find different instances harder depending on the quality of the group they ran them with.
What is clear is that the KUS bonus boss is the hardest thing in flashpoints atm. I personally welcome this; especially as it drops another Columi headpiece, which makes it worth the trouble. I believe this boss to be working as Bioware intended.
I agree, I believe this bonus boss was intended to be a very tough fight on hard mode because 1) It drops the head piece but 2) it's a new encounter.
People are so used to "zomg dps nao!" fights to beat enrage timers that anything that doesn't include an easy boss with a lot of health is too hard to comprehend.
Let me make this very clear: Enrage timers are a crutch mechanic that were obviously used for initial end-game content.
As the game comes out with newer content, I expect enrage timers to become "rare" for lack of a better term. There are much better ways to create interesting fights that are difficult without adding an enrage timer: Two perfect examples of this are KUS and D7.
KUS is a finely tuned fight that rewards good positioning, good reaction times (You need to use crowd control abilities liberally) and is somewhat of a gear check (Requires decent geared tank and healer along with DPS that can burn adds fast.)
D7 Final Boss is a fun fight that just requires a good healer that knows how to position and move, as well as DPS that understand how to do the same (DPS and move).
Both instances are the most fun I've had in a flashpoint since Red Reaper Boss fight (And possibly Collicoid War Games)