I really like LXQt for VMs. It is lightweight and fast enough to provide a very snappy environment, even beating out something like XFCE. With LXQt I get the minimally viable desktop environment with a panel, notification handler, etc.
Though most recently I have been using XFCE specifically because its notification widget gives me more info in the preview.
LXQt 1.4.0 released (lxqt-project.org)