These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Best Boss Ever: As brutally difficult as Mecha-Death is, he still has a cool design and is very well-animated, has a large repertoire of attacks, and the ensuing brawl hurls the two from one scenery piece to another.
Breather Level: Maybe not so much, since there are still robots and police after you, but The Mall. You simply have to destroy almost all the objects. You'll continue getting a consistent supply of'ON FIRES', and an easy 1Up with the power guitar that this level is considerably easier than most others.
Crazy Awesome: Dad full stop. We're talking about a guy who routinely destroys things in increasingly hilarious ways, can do his day's work in minutes, fires lasers from his eyes, blasts his enemies with The Power of Rock and more.
If you wail on Prez Boss as hard as you can in the Battleground before he gets away, then you can skip trying to manage the police, bombs and gatling gun/laser and just attack the cockpit directly and finish the fight in about a minute or two.
That One Attack: technically it's three attacks, but it's entirely possible for the Final Weapons to fire rockets, lightning ball and laser beam in quick succession. The resulting barrage is extremely hard to avoid in its entirety.
The regular Final Weapon's minigun is much harder to avoid than X's laser. Thankfully it's also significantly easier to kill.
Mecha-Death's most dangerous attack is his instakill clash, but the hardest to avoid is his charge, because he can do it several times in quick succession (sometimes longer than you can remain safe in the air).
Rajinmaru's lightning attacks. Especially in Tower of Destiny mode.
Sakupen's teleport and flame attacks. The other two at least have a huge amount of build-up to let you know they're coming, but the former has no tell at all and can easily hit you when you're up close and trying to deal damage, and the other only has a short tell, and is pretty much a One-Hit Kill if it connects.
That One Boss: Unsurprisingly, there's quite a few of them, especially on Hard Mode.
TRN-1 can be difficult, thanks in part to the high damage output he dishes out. Just one hit from most of his weapons can wreak havoc on your life bar. You get very limited time for damaging him and you have to jump to do so, limiting that time even further. Because of this, the battle tends to stretch over a pretty long period and you're bound to make some screwups - notably, it's all too easy to jump up to attack him, and then end up landing on his buzzsaw.
Ootkee, at least until you figure out that you have to play a reverse game of Donkey Kong.
Batt-Bot. Like Mecha Death on the main page, he has a large repritorie of attacks and no clear pattern, he can kill you quite fast, moves fast, and has high health values. However, there's plenty of items to hit and police reinforcements coming, so you can (and on Hard, pretty much must) charge up your "on fire" mode before attacking him.
Both Final Weapon and Final Weapon X, until you get the hang of their MANY attacks and the ways they can combine them to kill you (they tend to attack the ground in order to make you jump or knock you up, and then attack the air to finish you off). Even more so on Hard Mode, thanks to the rocket cops. Both are hard in their own way - Final Weapon's special attack is more difficult to fully dodge, but Final Weapon X is harder to take down due to the shield around Prez. Boss.
And to add insult to injury, if you lose, you're booted back to the start of the area regardless of how many lives you have, with Sakupen nowhere in sight. If you want a rematch, you have to quit to the main menu, skip through the opening stage and cutscene, and then run over and chase down Sakupen again before you can have it. Did we mention how annoying it is to have to chase him every single damn time?
And finally, Mecha Death. See SNK Boss on the main page as for why he so damn hard.