general is outdated use utf8mb4_unicode_520_ci if available otherwise unicode is fine
You should use utf8_unicode_ci for the database collation and utf8 for the database character set. The utf8mb4 character set and collations are inefficient compared to utf8, and applications that store raw emojis (such as WordPress) will manually set the utf8mb4 character set and collation on all the tables/columns it creates.
(FWIW, if WordPress required the PHP mbstring extension, it could use mb_convert_encoding() to store emojis as HTML entities, which eliminates the need for the utf8mb4 character set and collations.)
