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.