Favorite: KDE.

Least Favorite: Gnome, but I've not tried a ton... It's slick but way too opinionated for my tastes. I could change most of what bothers me about it with add-ons and mods, but KDE gives me the controls I want out of the box, and lets you do damned near anything you want before you even start installing mods (I don't understand the person who thinks it's too limited and Gnome is more flexible, but different priorities, I guess)

First DE: KDE on XanderOS... To be fair, the name sounded cool to young me and I didn't know much of anything about the different Linux distros yet. And I think Ubuntu either wasn't around yet or hadn't gotten well enough known for me to have spotted it initially

Honorable Mention: Unity... Love it or hate it, Ubuntu was trying something actually interesting and I think it's a shame they didn't stick with it longer

Current DE: KDE

Potential Future DE: not sure, KDE does everything I want at this time, but I might give Cosmic a try, and/or a tiling WM, though I suspect I won't care for it (I'll give it a real shot to convince me, I just don't see the appeal)

Longest used DE: KDE

Current distro: Mint with KDE shoehorned in, though will be OpenSuSE Tumbleweed in a couple hours (I love Mint but I'm tired of fighting with old packages and drivers, and suspect that might be the cause of graphics stack crashes that have plagued me... Also because I'm tired of waiting for KDE 6 for years now and it'll be probably mid next year still before it's available on Mint, and even then still only as an afterthought)