I have to say the best biceps exercise is that one that is perfect for the activity one is engaging in.
How does a boxer train?
How does baseball player train?
How does a basketball player train?
How does a swimmer train?
It’s important to take into consideration specificity and train the muscle group so that it’s adaptation is perfect for the activity.
there are really only so many ways in which an elbow flexes ……
What I like to do is vary the loading itself. Dumbbells, cables, machines with specific cams and tubing, to name a few, all load in different ways in addition to focus on concentric versus eccentric loading.