Internet Explorer.

Unless you're a startup and don't have to give a damn about what browsers your users use, IE marketshare is just too big to ignore. And depending on your audience, IE can mean anything from IE11 all the way back to IE8.

Theoretically IE is irrelevant now that Microsoft has started publicly killing it off, but you'd be surprised at the number of corporate users that are still on Windows XP (or maybe Vista/7) running IE 8 and 9.

Of course evergreen browsers are less of a problem but amazingly a lot of the same users who are stuck with IE also manage to have extremely outdated versions of Firefox.