There is no singular best type of bicep curl that will be optimal for all lifters. Barbell and dumbbell curls both have pros and cons for building the strength and size of your biceps. Barbell curl variations include the EZ bar curl, which gives you some of the range of motion benefits of a dumbbell curl whilst allowing you to load a heavier weight. Seated or preacher curls are a useful way to isolate the biceps in either a barbell or dumbbell curl, as they eliminate the legs from assisting the lift. In a 2014 study, dumbbell concentration curls, where the elbow of the working arm is propped against the inside of your knee, were found to be more effective at engaging the biceps than other variations.  Experiment with different variations to find what you get the best results from and which you enjoy the most. The most optimized workout program will do little for you if you dislike it so much that you aren’t consistent. Finding what works for you and being consistent with it is the best way to get the best results.