一番楽しいのはどんなとき。

number one; first; first place; best; most; game; round; bout; as a test; as an experiment; by way of experiment; by way of trial; tentatively; song (e.g. in Noh); piece

enjoyable; fun

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

what; what kind of; (with -temo, demo, etc.) any(body, thing, means, etc.)

regular (stops at every station) Jouetsu-line Shinkansen