I tried them all and other Ubuntu flavors too, and from my experience I can tell the following:
Kubuntu and KDE Plasma in general the heaviest of them, it make my CPU hot faster than any other desktop environment.
Ubuntu Mate by far is the lightest between them, but if feels old compared to other two, and its development is the slowest between them, so you will not get new features with every release, it aims to continue the Gnome 2 for people who used to it and don't like to change for something new.
Ubuntu on the other hand is heavy compared to Ubuntu Mate, but I feel Gnome is way better in managing resource usage more than KDE Plasma, and here I mean usage in general not when cold boot, to make it more clearly, when you open Kubuntu it may take about 600 mb RAM but usage will be sluggish and RAM will rise fast, but with Ubuntu it will start about 1 GB, but everything will be smooth all the time, CPU usage is better in Gnome (Ubuntu) than Kubuntu (KDE Plasma) all the time.
So if you look for lightest it will be: Ubuntu Mate > Ubuntu > Kubuntu.
Personally I am in love with Gnome desktop, but you have to know it is made for shortcuts users not for mouse or touchpad, if you want to use Gnome features in a traditional way (with mouse) I recommend you check Ubuntu Budgie, it takes Gnome and make it lighter and traditional, it combines Gnome and Mate somehow and add more features with every release.