首 is one of the oldest pictographs in the language: the oracle bone draws a human head with hair and a single staring eye. Three meanings collapse into one sign — head, neck, and "the foremost / chief." Anything that sits at the top of something — a leader, a capital city, even the count of a song — pulls the same character.

Korean reading "su." 首都 (sudo, capital city), 首席 (suseok, top rank / valedictorian), 首相 (susang, prime minister), 元首 (wonsu, head of state), 首肯 (sugung, "to nod the head" = to agree), 首尾 (sumi, "head and tail" = beginning and end of an affair). Korean retains the abstract "foremost" sense of 首 powerfully — almost any "top X" official title in Korean uses this character.

Mandarin shǒu, 3rd tone. 首都 (shǒudū, capital), 首先 (shǒuxiān, first of all), 首席 (shǒuxí, chief / principal), 一首歌 (yì shǒu gē, "one song"). Mandarin uniquely uses 首 as a measure word for songs, poems, and short literary works — a grammatical role you will not find for the same character in Korean or Japanese.

Japanese on-reading シュ (shu) — 首相 (shushō, prime minister), 首都 (shuto, capital), 元首 (genshu, head of state). Kun-reading くび (kubi) — and here the meaning narrows: in modern Japanese, 首 means specifically "neck," not "head." 首が痛い (kubi ga itai) = "my neck hurts." Japanese also turns 首 into the everyday idiom 首にする (kubi ni suru, "to make their head [come off]") meaning "to fire someone from a job" — a vivid trace of Japan's feudal vocabulary surviving in modern HR slang.

Memory aid: a head with hair and one eye drawn together — the topmost part of the body, then "topmost" of anything.