KDE4. Plasma. The fattiest FOSS has to offer.

Worth looking at Trinity (KDE3 fork/continuation).

XFCE looks like it’s trying to go from lightweight to fattiest some upgrades too. Still, very elegant, and you wont notice that on new beefy hardware. But on ancientware, … best stick to LXDE, or even just openbox, or any other window manager, pretty much.

IceWM, calls itself a window manager, but it seems to cover all the main basics for a desktop environment feel.

There still be places to go to get the light, fast and shiny in FOSS, even if KDE went nuts around 15 years ago (whenever KDE4 was).