Iji brings us a trifecta of absolutely awesome bosses:
Asshead Asha the Assassin is disgustingly satisfying to kill, as he decides he's got some sort of horrible vendetta against you, and especially if he killed Dan. But the best part is using the Nuke on him in the first fight and watch him shit himself in terror.
His attack pattern in the second fight is also intimidatingly awesome. Rapid teleport, followed by close-up energy sword, combo'd to a wall-jumping Plasma Cannon or a huge jumping shockwave as he shouts "Just DIE!!" And you can only beat him with your shotgun, because it's the only weapon he deems too weak to dodge, which makes the fight feel like a true duel between rivals. His death here is also quite satisfying - the resulting explosion blows his head off and reduces the rest of his body to a bloody smear.
Annihilator Iosa, also known as "Iosa the Invincible", as she survived being hit with a planetbuster. And, as you will find once you get her out of that Berserker suit, the name is QUITE literal.
And how do you beat her, you ask? Why, you kick her in the face until her head slams into the wall, then crack her Nanofield while she's stunned. Then Iji finishes her off with a shotgun shell, or for pacifists/near 0 kill count, an assassin that agreed to help you stabs Iosa, through the head.
General Tor, who is fought under the haunting lights of a fleet warming up its planetbuster to the most awesome music in a game with a Crowning Soundtrack Of Awesome. And absolutely heartbreaking, as he wouldn't be fighting if the general population didn't have some homicidal obsession with ending the Tasen race.
His Beam Spam, Macross Missile Massacre and other highly explosive weapons also adds to the experience. And if you're too careless and allows him to charge three glowing shots, he combines them into his Wave Motion Gun, which is usually installed on battleships for use in the planet busting Alpha Strike and uses it to vaporize Iji in one shot.
Nevermind the fact that the Phantom Hammer not only kills you, but it blanks out all your gauges, text, and numbers on your HUD. It actually wipes out all your stats.
Try beating the final boss on Ultimortal and maximum charged. Makes it epic knowing that you beaten the final boss, on the hardest difficulty, while you made said boss stronger before the fight.
Tor himself is even shocked that you beat him with his shields fully charged and begs for mercy.
Plus if you're playing a Full Pacifist Run, you can use the resident BFG - the Massacre - which Ansaksie drops off for you shortly before the fight. Finish Tor off with it and watch him panic.
Also, you can survive the Phantom Hammer (well, dodge it, but it still counts).
For some, the most awesome of all awesome bosses is Proxima the Sentinel. This crazy titanium sphere likes to hover just out of sight, so you can't tell where its next attack is going to come from, and fire a huge array of weapons at you all at once. Usually, avoiding one attack means jumping right into another. And, unless you've concentrated exclusively on weapons skills, it's barely possible to damage it; you have to dodge and keep dodging until it enters its "charging up" phase and then you finally get to run up to it and give it a good, hard kick into an electric field. So satisfying.
Proxima can be kicked into the fields when it's using the Plasma Cannon attacks, and if you time it right, you can even make it fly itself into one of the fields in it's attempts to ram you. It's not very smart.