Which X should you use?
โ and โ๏ธ (thin and heavy "multiplication X") are the classic close-button and delete icons. โ and โ are ballot X marks: they mean "wrong", "no" or "not done" and pair naturally with the โ from the check mark symbol page.
โ๏ธ is the color cross mark emoji, and โ๏ธ is its boxed button version. For forms and to-do lists, โ shows a box with an X inside. โ (saltire) and โณ are plain diagonal crosses that work well in text art; for upright crosses, see cross symbols.
In math, use the real times sign ร instead of any of these. The multiplication symbol page explains ร vs x vs *.
Fancy letter X for names and bios
Circled โง, negative circled ๐ ง, squared ๐ and double-struck ๐ are letters, not icons, so they fit into usernames, bios and titles. ๐ is popular because it resembles the logo of the social network X. To restyle a whole name, try the fancy text generator.
๐ฝ is a regional indicator letter. Two of them side by side form a country flag; on its own it shows up as a single letter, often inside a box. All of these come from Unicode math and enclosed letter blocks, so screen readers may read them oddly. Use them for decoration, not for important information.
X look-alikes that are not X
Some characters only look like x: the Greek ฯ (chi), the Cyrillic ั (ha) and the Roman numeral โ ฉ (ten, see Roman numeral symbols). They are fun for special effects, but search engines, spell checkers and login forms treat them as different characters, so a username typed with ั will not match one typed with x.
To type โ๏ธ quickly, open the emoji panel (Windows + . on Windows, Control + Command + Space on a Mac) and search for "cross mark". On phones, search the emoji keyboard for "cross".