As others have said, it completely depends on your child and you should try out each option to see what works best for you.
My son went to MyGym from about 6 months to 2.5 years and loved it. Especially once he started really walking and climbing, he just loved having a space where he could run wild and tumble all over. I'm sure the experience differs from place to place, but we loved our MyGym instructors and had such a great experience there. It was the perfect mixture of free play and structure for us. We tried Gymboree, but I didn't care for it. We also did a music class for a while, and while I loved it and thought it was adorable, my son was bored and just ended up running around and not paying attention each time we attended. Once you do some trial classes, I'm sure you'll know what's best for your child.