UTF-8
The dominant Unicode encoding — variable length, 1-4 bytes per character, backwards compatible with ASCII.
"a" = 1 byte. "ä" = 2 bytes. "猫" = 3 bytes. 🐱 = 4 bytes. Saves space on English text, handles everything. The modern standard everywhere: web, files, databases. UTF-16 (Java/JS internally) and UTF-32 are alternatives that lost ground.