Threading is completely broken, filtering is broken, compose screen is unusable.
At previous companies I've had to use gmail but was able to use a sane email client via IMAP so it was almost ok (although still somewhat broken as gmail doesn't handle IMAP correctly). At current work they disable all access except via the unusable gmail web interface. So definitely gmail is the worst I have to put up with everyday.
jira would be a distant second, but no comparison.