What are you all using for your Slackware VMs?

I've only used VirtualBox. Unfortunately, its OpenGL virtualization is horrid even if you turn it on. I've found that compositing is invariably completely broken. That means no KDE desktop effects, no Compiz, no Compton, etc.

Is there anything better?

I've been using VirtualBox for, oh, some years now and really haven't experienced what you're describing. I do use the binary installation, plus the now-Oracle Extension pack and the Guest Additions packed in with the Binary (the ".run" one). It gets installed in /opt, works just fine and if I want to remove it completely there's an uninstall. Gotta admit, I don't use KDE, real or virtual, in favor of Xfce (which works just fine in a virtual machine). Right now the only virtual machine installed is Win7 (for the rare instance of needing it plus the usual Patch Tuesday stuff); 3-D effect work fine.

Maybe give the "*.run" a try and load up the Extensions and Guest Additions?

That is, unfortunately, what I've been doing.

I've used VirtualBox and VMware Player, now I mainly use qemu.

I use virtual machines mainly to test Slint installers that (as genuine Slackware ones) have no GUI, so that's a very different use case: I'm not interested in bells and whistles, but by virtualization of BIOS or EFI firmware. I didn't try qemu 2.0 yet but will soon.

in August of 2011 I used this to turn on compositing in Virtualbox > KDE guest...
It may still work?
This should be done on the guest, of course.
and change
OpenGLIsUnsafe=false to true

in
[Compositing] section.