As an avid player of P5 and a semi-avid watcher of Riverdale, I have to reluctantly give the vote to Betty, if only due to the fact that (as it's been previously mentioned) fuccboi Akechi is not in actuality a Detective Prince, but more like the exact opposite (like, an Ace Defective or something), in that virtually all the crimes he "solved" were in effect caused by him. The one ounce of sleuthing he does effectively do on his own is so inherently obvious (the identities of the Phantom Thieves) that anyone w/ half a brain (including a Student Council President & a NEET) could figure it out within a week. He also instantly gave away that he knew by responding to a cat.
Betty on the other hand, is not actually a Detective but is an amateur journalist for her school newspaper, which arguably has given her more experience in information gathering than Pancakechi, effectively making her the only logical choice.