I like Superbiiz.com. You can select what parts you want, they will build it for you, install the OS, and give you a 1-year warranty. You can also use their "system builder" tool to pick things and have them build it, but the system builder tool has fewer options to pick from than their whole inventory.
And superbiiz.com is very cheap.
I have been making AH computers for about $350 for the computer + $100 for Windows + $140 for the graphics card + $100 for the monitor. They are fine for me, but I do basic 1080p 60 Hz monitors, not VR, 4k, or 144 Hz.