Lord Blackburn becomes very easy if you have a hero. Gerald can counter his attacks even though they are AOE, and the true damage does wonders against him. It actually puts Oni to second place. Thankfully, as long as the hero is on the move, they cna pass through Blackburn’s AOE stun unaffected, so you can almost perpetually stall him. I never see him get even halfway to the exit with a hero.
Althouh not technically a boss, I’d consider Cerberus to be one of the most threatening things the game can offer. What waves stick more in your mind than the ones that spawn a Cerberus? They demand serious preparation and your full attention.
But enough dodging the question. Most bosses in KR1 are easily countered by having a melee hero. Without one, Blackburn and Moloch in particular becomes more threatning. I’ll have to agree with Ninja on Blackburn being tougher, as he has an arguably more powerful ability with greater AOE, along with High Armor.
KRF has Xyzzy, who is still rather easy just because of how many towers you can build in that stage with all the gold available. But, this extends to Vasile and Leviathan as well, as a chokepoint can be built on the left part of the stage to counter Vasile in his first form, and the second form falls easily since the plethora of mages built there to counter previous waves will do immense damage to him. As fir Leviathan... if it can deal damage, it will counter him. So, ultimately, I’d say that Umbra is the hardest KRF boss, and the most dynamic. Because of the amount of preparation I do prior to fighting him in his second phase, I have to conclude that he puts up the most fight.
In KRO, I also agree with Ninja - Godieth doesn’t have much of a weakness. His unremitting attack rate coupled with his high damage makes him unblockable for more than a couple of seconds. His health pool is much higher than Baj’nimen (although he can heal 2k health per cast, technically putting his total health to an extra 6,000 more, I believe) or Bram. The enemies in his company are much faster, distracting fire from him.
Overall, I think Umbra still takes the cake for the toughest boss, with Godieth as a close second for me.