bitflipper
"Essential" implies that music literally cannot be made without it. yorolpal has never used hyperbole in the past, so I can only conclude that it's the lack of Maag EQ4 that's to blame for me not finishing any projects lately.
The temptation to have fun with your post is great... but i will resist.
There probably is nothing, other than my guitars, that I consider "essential" per your definition (which is a good one, accurate even.)
And there are very few tools, hardware or software, that I'd claim make my job easier. In terms of filters I'd say that I'd have to work a lot harder if I did not have Zynaptiq UnFilter and MAAG EQ4... in fact there are a number of tricks I could not even do without these two tools. They span the chasm that separates artistic and technical, and they let me shape the spectrum EXACTLY as I wish. There is no analog (pun intended) to UnFilter, not in hardware, and not yet in software. It isn't necessary for every production, but when I need it I need it. And while there are both hardware and software analogs to EQ-4 I choose to use EQ-4 because it works as I expect it to work.
Essential? No... I could still make recordings without them. Important? You bet. In fact I'd be perfectly happy if no one else ever purchased either, as they would then become my secret weapon<G>!