I agree. Your question is a bit ambiguous. Do you mean the best bicep mass building exercise or the best exercise with biceps that builds mass?
So, if you want big biceps, I'd say barbell curls is the best thing to do, obviously. It's direct stimulation.
But, if you are asking more of how to get a bigger arm because of overall mass question, then chinups and heavy rows are more the ticket. Also, you arms will grow even if you don't work them directly. Heavy squats, bench, and deadlifts make your arms grow too, indirectly.
Big lifts mean big arms. That's what chillen is getting at, I believe. And I agree.