Which check mark to use
The light check mark (U+2713) suits running text and lists, the heavy check mark (U+2714) stands out in tables and labels. For to-do lists, use the empty ballot box (U+2610) for open tasks and the ballot box with check (U+2611) for finished ones, or the ballot box with X (U+2612) for tasks you dropped. The cross marks (U+2717 and U+2718) are the natural opposite of the tick for yes and no columns.
The square root sign and the large circle are also used as marks in some countries: the circle means correct in Japan and Korea, where a check mark can mean wrong.
Typing a check mark
In Word you can use Insert, Symbol and the code 2713 or 2714 with the font Segoe UI Symbol. In Excel and Google Sheets, copying the symbol from here is the quickest way, and conditional formatting can color the cells. In HTML, write ✓ or ✔.