Kettlebells: These require a surprising amount of coordination and finesse. The athlete must learn to use only enough energy during exercises like the snatch, clean and press, and Turkish get-up to keep his or her technique in check. Trying to overpower the kettlebell leads to sloppy technique. Bells build focus and discipline, both traits that any serious athlete must have.
Barbells and dumbbells: The free weights will always be the king of the weight room for me, for the simple reason that you can move the most weight with them. But that realization comes with an equally big responsibility to learn form and earn your numbers.
If you want to be barbell-strong, you simply must learn proper technique on the squat, power clean, deadlift, bench press, and bent-over rows! These are all surprisingly advanced exercises, and plenty of otherwise-advanced athletes never developed the strength to do them properly.