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.
YMMV: Dad Series
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.
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.
That One Boss: Unsurprisingly, there's quite a few of them, especially on Hard Mode.
TRN-1 can be difficult, if you don't know what you are doing, thanks in part to the high damage output he does.
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 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. Even more so on Hard Mode, thanks to the rocket cops.