I built this computer for a group at a research institution. Both reasonable CPU and GPU computing power were needed. Money wasn't an issue.
The primary goal of this system is to run a plane wave density functional code VASP. Since it will run 24/7 I particularly focused on quality, endurance, and performance. With GPU computing coming online as a major player and CUDA compilers being released for many languages I decided to add a Tesla K40 to the system. Otherwise the RAM and CPU are the main components of concern. I decided to only put in 64gb of DDR4 but the motherboard can support up to 512gb for further expansion. As for the CPU's I chose two Xeon E5-2630's. The machine has run now for months without any hiccups and the group I made it for seems pleased.
As for the other parts I usually erred on the side of endurance and reliability. I chose Enterprise 2 Tb HDD's, a 256gb Samsung Pro, Phanteks large air CPU coolers, and the Corsair 750D Airflow edition case -- mainly for good looks and expandability. A Tesla K40 sli setup may be added in the future to add some computational gravitas.
Since there is interest in an upgrade, I've been looking at a machine that possibly comprises of dual 22 cores and Nvidia's new Tesla P100. Although I'll probably wait for Intels new CPU's and Zen to come out first.
I put two of these in a workstation system and they work well and have run for many months straight with a demanding computation load.
I good dependable CPU cooler, although it is a little two large. Otherwise I recommend it.
Fast and reliable SSD.
A pretty case with all the expandability you'd need. Although I had problems mounting my dual-socket motherboard.
A good PSU for systems that could/will take a lot of power. It has a 10 year warranty, so where reliability is concerned it checks that box as well.
Decent and working CD Writer.