What ISO image should you download to install full featured Debian Linux?

All Debian ISOs are "full-featured"; you can use any of them to install any Debian programs. The KDE and GNOME ISOs are probably best if you want a desktop system, and the netinstall is a better solution for servers and systems you need / want to customize heavily. However, nothing stops you from, say, installing KDE on the netinstall system, or installing Apache on the GTK system.