brain-teaser; brain work-out

I'm thinking for the context of a "life decision" rather than a riddle. Let me add that to the question.

I think you want something like dilemma or choice in your example sentence, instead of decision. In the current phrasing I assumed you meant a court decision was hard to figure out until I read your later comments. You could also phrase it "I have to make a decision, and it's..."

"head scratcher" is one way to describe a puzzling problem. "mind-bender" could work for something perhaps confusing to accepted rules/expectations.

Thanks, @1006a, I updated the phrasing on the sample sentence.

@Tom22, I like "head scratcher" a lot actually since it is catchy and memorable and works well as an opposite to "no-brainer". I'd vote for it if you added it as an answer. Some of the other answers are great, too, but this one I like particularly.

" almost want to say, "...it's a real brainer." but that just sounds silly." I love this actually, although not in the sample sentence

"...it's a real brainer." while not grammatically correct, I think this is more than perfectly understandable as it directly follows "no-brainer". If grammatical correctness (or professionalism) is not your main focus; I see no reason to avoid using this.

In an operating room: "After all, it's not brain surgery ... oh wait, it is." At NASA: "Look, Smithers, it's not rocket science ... oh wait, it is." But those are very situation-dependent comebacks, of course.

Around my block it's called a "hairball".

I like "whole-brainer"

To quote the King of Siam in the original film "Anna and the King of Siam": "Is a puzzlement."

It's a real sticky wicket.

a real bear , or some other B word ... ( this is a dupe; I've made that exact comment before )

I gotta say, as melon scratchers go, that is a honey doodle.

I like "brain-drainer" in your very specific instance. 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.