Which speech sounds are usually the hardest?

Sounds that need fine tongue-tip control and steady airflow: fricatives (s, z, f, v, sh, and *th* /θ, ð/), affricates (ch, j), and liquids (r, l) — plus consonant clusters/blends (st-, spl-, -nts) and final consonants. In English, *th* and the *r/l* contrast are common late ones, and clusters are a big challenge because English has so many. This reflects oral tone, mouth structure, motor-planning (childhood apraxia is more common in Down syndrome), and hearing — not how much the child understands.