The joker is a lot smarter than people give him credit for. I think the Riddler's pride would get him killed. The Riddler can be predicted, and countered because he wants to show off how smart he is. The Joker is almost impossible to predict.
The Riddler would have a perfect, master plan, but the Joker is too unpredictable. Batman is smarter than Nygma, and even Batman can't predict what the Joker will do. Joker would win - the only question would be whether or not the Joker would spare Eddie after he'd won. Even if it was a fight to the death, I could see Mr. J thinking it'd be funnier to let Riddler walk away defeated.
Is there prep time? Or a random encounter? I personally dont think riddler would be able to "riddle" joker because he is just too crazy and unpredictable So ima go with joker My favorite between the two is joker Thats a lie he might be my overall favorite villian, i also like reverse flash and sonestro