This was probably the very first lesson that I learnt as a natural haired girl - avoid sulfates, they dry hair out! However, if you look deeply into the subject firstly, not all sulfate-containing shampoos are as harsh as each other; secondly, not all sulfate-free shampoos are gentle - confusing, I know.
According to the Science of Black Hair, "Ammonium lauryl and laureth sulfates (ALS) are harshest and tend to be the best cleansers, followed by sodium lauryl and laureth sulfates (SLS) respectively. TEA laureth sulfate and sodium myreth sulfate are gentler sulfate detergents."
Importantly, some sulfate-free shampoos contain so many cleansers that although each one on its own is gentle, in combination the shampoo becomes more harmful than a sulfate containing shampoo!
To stress my point look at the ingredients listed on the right. These are all in L'Oreal's Everstrong Hydrate Hair Sulfate-Free Shampoo. Everything in bold is a detergent or "surfactant" to use the fancy term and I don't even know what the rest of the stuff in there is; this bottle is pumped full of chemicals. With a sulfate-free shampoo this packed with chemicals you may as well just purchase a regular sulfate "moisturising" shampoo. It will cost far less and won't dry your hair out that much either.
On the other hand, Kinky Curly's Knot Today only has Cocamidopropyl Betaine and most Aubrey Organics shampoos have the even gentler Decyl Glucoside. These are good sulfate-free shampoos. Three gentle cleansers you might want to buy: