A no-brainer is "something that requires a minimum of thought" (Merriam-Webster). I could use some help with a catchy way of saying the opposite.
"I have to make a decision and it is definitely not a no-brainer, in fact, it's a real ___________."
I almost want to say, "...it's a real brainer." but that just sounds silly.
You could always say "...it's a difficult decision" but that's pretty bland.
Someone suggested brain-teaser in the comments, but I'm thinking of the context of a life decision like, "Should I accept this job?" or "Should I get married?" -- something of that order -- rather than a riddle.
I searched for 'antonyms of no-brainer' online but the first three hits came up empty.
2 Informal (originally US). A perplexing problem or question; something which causes bafflement or puzzlement.
Although OED says that this term is originally AmE, it is certainly widely understood in BrE.
Definition of conundrum, from Merriam-Webster. Only definition 2b, which I bolded, applies to your question, but conundrum is a fun word (at least IMO), and having three so different definitions adds piquancy to the word.
1a. riddle whose answer is or involves a pun (as in “Why didn't the lost hikers starve in the desert? Because of the sand which is there.”)
2a. a question or problem having only a conjectural answer •… the political conundrums involved, particularly the problem of how the richer areas … can be made to subsidize the poorer. — Douglass Cater
2b. an intricate and difficult problem •He is faced with the conundrum of trying to find a job without having experience.
If the OP insists on brain in the answer, this won't work.
Brain Buster (N) A particularly difficult problem or puzzle.
See also: "Brain teaser", "Head scratcher", "puzzler", "quandary"(N. A state of perplexity or uncertainty over what to do in a difficult situation. "X was in a quandary") and, of course, the other answers like "conundrum". (a favorite of mine)
At Work & in other official-speak, I usually use the phrase: non-trivial
It's a wonderful way to tell people, especially managers that what they are asking for, requires more thought than they have given, without insulting them.
This phrase is used quite a bit in Academia, especially Mathematics, where trivial & non-trivial are used to indicate the complexity, especially of proofs of various theorems and conjectures.
A “tough nut to crack ”, which is defined by the Cambridge English Dictionary as
a problem that is very difficult to solve …
You probably wouldn't use the word “real” with this.
If one is in a literary turn of mind, you might describe it as a three-pipe problem.
This is derived from usage in the Sherlock Holmes stories where the number of pipes relates to the complexity of the case.
That's clearly a toughie (or perhaps a toughy), short for tough problem. Akin to sweetie, goody.
The opposite of this task is a no-brainer may be this task is rocket science.
Now, I have only seen "rocket science" in the negative ("this is not rocket science", equivalent to "this is a no-brainer") but Shania Twain seems to concur.
May I suggest 'noodle-baker'? As in, "What will really bake your noodle later on is would you have broken it if I hadn't mentioned it?" (The Matrix, for those wondering).
It kind of fits: it's either a no-brainer or a noodle-baker.
Great to see lots of choices out here, hope you find one that suits!
Surely, the answer is obvious: 'Brainer'.
Obligatory Peep Show joke:
Jeremy: She's trying to shaft me, Mark! You've gotta dump her, it's a no-brainer.
Mark: It is not a no-brainer. I'll have to think about it. It's a brainer. It's a real brainer.
For the benefit of non-native English speakers, it's worth noting that the humour in this joke is derived from the fact that 'brainer', despite being the obvious antonym for 'no-brainer', actually sounds kind of stupid and out-of-place.
A "poser" seems like it would be what we're looking for here.....the definition is "a difficult or perplexing question or problem".
I propose flummoxer, from the verb to flummox:
Jean Strouse reflects:
There is greater intensity and more imagination in flummox than in its near relatives, baffle, perplex, confuse; the comical sound of the word adds to its strength—though the same can be said for discombobulate. Flummoxed conjures up a figure in momentary speechless paralysis, whereas discombobulated suggests a human contraption coming all to pieces.
An opposite to requiring no brains is to require at least one, and possibly several brains.
"I have to make a decision and it is definitely not a no-brainer, in fact, it's a real ten-brainer."
The choice of number admits to some subtlety.
To fit the meter of the phrase, the number has to be single syllable. For instance a seven-brainer sounds clumsy.
In order to avoid being misconstrued, two-brainer should probably be avoided as it might be interpretted by the listener as a request for their help with the problem, unless that's what is wanted of course.
Three has a difficult mouth position to start with, four sounds like fore, allowing confusion with fore-brain, which is not intended. Eight with its unstressed start is also a poor choice.
Any of the numbers 5, 6, 9 and 10 would work. In picking a number that's not one, ten is perhaps the most obvious, and sufficiently larger than one that it suggests you are being rhetorical, and not specifying the size of the committee that is to deliberate.
This may feel different in other languages, or for speakers whose first language is not English.
One not yet offered:
When I was a student at MIT, an easy problem was called "obvious". If they wanted to emphaszie how easy it was if you gave it even a small amount of thought, the professor would say it was "intuitively obvious". If they really wanted to annoy the poor student who could not grasp such an elementary concept, they would say it was "intuitively obvious to the casual observer".
Thus "intuitively obvious to the casual observer" is what MIT students use to sarcastically describe any problem that is neither intuitive, obvious nor inclined to make one feel casual about tackling it.
Whole-brainer as opposed to no-brainer parallels really nicely. (Documenting @Barry Franklin's comment as an answer.)
"I have to make a decision and it is definitely not a no-brainer, in fact, it's a whole-brainer."
It is very similar to "brainer" which the OP (me) originally thought and others felt was good, but this parallels even more nicely with "no-brainer". Even though it's not very common, it works really well in the sample sentence.
Obviously, the only correct answer is "yes-brainer".
"Uh oh! We better pause and think here; this decision appears to be a yes-brainer."
(This answer documents @Pauan's well-accepted comment as an answer and @Kaz's clever usage of it.)
I would say use your original idea: it's not a no-brainer, in fact it's a real brainer. It better accomplishes that "fun and catchy" part of your question than the serious answers above. I would use it in the combination with the "no-brainer" part to avoid confusion and make the contrast work better, though.
YMMV - I am not a native English speaker.
I like brain-drainer in your very specific instance.
"...it is definitely not a no-brainer, in fact, it's a real brain-drainer."
It's invented for the purpose, highlighting the fact that we don't really have a set phrase for "opposite of no-brainer"; it connotes the fact that instead of requiring "no brains" you will have to use up your whole brain to get the thing done; and it rhymes with "no brainer", which provides a bit of contrastive symmetry for your usage.
(This answer documents @Roni Choudhury's comment where it belongs).
Not sure if pop culture references are permitted, but The Simpsons way of saying this would be a 'dilly of a pickle'.
The character Joey from the show "Friends" has used the work thinker to refer to something like this.
This takes place in Season 4, Episode 16 (The One With The Fake Party). Joey thinks Chandler has made a joke (when he really hasn't), and asks Chandler what he said. Chandler tells him and Joey is confused because he doesn't get the joke (in fact, it's not even supposed to be a joke). Joey just shakes his head in defeat and says "Now that's a thinker!".
I haven't seen the word being used in this sense elsewhere, but you wanted "fun and catchy" and (in my opinion, at least) there are few things funnier or catchier than the dialogue from that hit comedy show.
I would say "it's a real brain-burner", implying that it requires a lot of time to think about the decision.
I couldn't find a formal definition for this phrase, and googling it seemed to only turn up board-game related results. I didn't realize the phrase is typically limited to that domain, I personally use it wherever it applies.
This is a job for intellects vast and cool and unsympathetic: HG Wells: https://fantasiesofpossibility.wordpress.com/2016/12/09/intellects-vast-and-cool-and-unsympathetic-the-war-of-the-worlds-by-h-g-wells-1898/