Actually, in terms of KR canon, Decade is the most powerful. More specifically in his Fury Form(I know the official name, but this sounds cooler.) Because he was able to single-handedly destroy every Rider. The only reason Kivala won is because Decade let her win.
Canon? Decade is made up of nothing but plot holes. It doesn't get to use that term. This is why there are people that honestly believe Accel Mode can keep up with Clock Up, even though Accel goes the speed of sound and Clock Up dodges bullets.