No Budget

Hypothetically speaking, if someone was to have a grand amount of money to spend on a rig, what would the best place to start be? The motherboard? I've been trying to find the motherboard that would offer the most in terms of supporting high end parts while offering options for expansion later on (not a limiting number of slots). I was wondering if any of you had input on my search and know of what motherboards (or other parts) would be best to look at. I'm curious as to what's commercially available.

I think server grade motherboards might fit what you have mentioned. But i forget if they are commercially available or specifically designed 

the BEST place to start is what you want to DO with your rig.

-gaming/web browsing/music/movies/streaming

-Photoediting/video editing/3D model and rendering

what will you be doing?

Start with the CPU, then look into a motherboard that will have the best potential for it and continue forward.