That depends where you want to go with your system, but Unity seems to be one of the most obnoxious Desktop Environments when trying to get it to look different from its default-look.

Apart from that, if you're going to replace most of the DE-functionality with dedicated components anyways, you'll want to start out with a more minimal DE, if possible, for the sake of resource usage.

Arch Linux.... or another rolling distro.

You get the software releases as quickly as possible.

This empowers you to work with the latest releases and also forces you to be more conscious of what is supported by which software release. In which case you spin up VMs to test out all the other distros / software releases for compatibility concerns.

My 2 cents... Sometimes Ubuntu 14.04 will be perfectly fine for whatever environment you need...

Most people here are replying Arch. That's what I keep coming back to. (over and over again)

Arch maybe a better starting point than Ubuntu