TDE (for those who haven’t encountered it before, the Trinity Desktop Environment forked from KDE3 more than a decade ago). It might not be the flashiest or the newest, but it has a decent selection of features and applications, and presents a traditional desktop environment whose interface doesn’t get changed for the sake of change. In other words, it stays out of the way and lets me get things done.
(If I’d liked Gnome 2 better than KDE 3 rather than vice-versa, I probably would have gone for MATE instead.)