This may not be the most important biceps exercise, but it's my favorite. For maximum size and overall biceps belly mass, do standing barbell curls, but for maximum peak, hardness and split, nothing beats the concentrated pump from preacher curls. It packs my biceps so hard that they end up feeling as solid as forged steel. I credit preachers more than any other exercise for the hardness and cannonball shape of my biceps and for the deep split in their peaks.
Use heavy weight, but stay tight and don't fully extend your arms at the bottom; you could tear a muscle and/or hyper-extend your elbows. Do not leverage the curl upward by pulling back with your body, no matter how much you're tempted. Watch your biceps pump and harden as they flex. It's a great motivator, and it helps keep your mind on how the muscle, not the ballast-effect of your body, functions. At the top, get a peak-contraction squeeze. Maintain a moderate pace, the same during the extension as during the curl.
For every other workout, superset preacher curls with another two-arm curling movement. For the most immediate mass production, I do four sets of eight to 12 reps each.