Title case

Title case capitalises most words, sentence case capitalises only the first word and any proper nouns, and all caps shouts. YouTube imposes no requirement either way and runs no validation on the field. What capitalisation does is set register: sentence case reads calm and personal, title case reads formal or editorial, all caps reads urgent.

Pick one and hold it across the channel so your titles look like a set when they sit side by side in a browse row. Very large channels exist at both ends of the range, which suggests the words themselves carry far more weight than their capitals.