You should give KDE Neon a go. It's Ubuntu LTS with the latest KDE, so you've got a stable base with a current UI on top. KDE4 drove me over to Cinnamon (Linux Mint) for a while, but I'm back on KDE5/Plasma, and I've realized I should have gone back earlier.

It's tight, streamlined and has a pleasantly small base install that doesn't get in your way with badly chosen default apps.