PC Components

A brief guide on PC Part selection.
Having good specs does not necessarily make a PC part good.
Always read reviews.

Monitors

Go IPS every time. If you're really rich, you can go OLED.
I prefer 4K over 120 Hz but if you can find a good deal on QHD 120 Hz that's good too.

Peripherals

PC Parts

Case

Go with something ~$100 with looks that you like. Gamers Nexus does airflow reviews but as long as it has 2 front fans with a clear area for intake, it's good enough for me.

Heatsinks

Just go Noctua if you can afford it.
NH-D14 or NH-D15 are excellent. They'll last 10+ years.

Fans

Again, go Noctua. Their fans will last 10+ years.

Avoid all sleeve bearing fans.
They'll make ticking sounds or perform poorly at low speeds.
Try to find PWM fans if you can but most motherboard will support DC control.

Arctic fans are supposedly good budget fans but I don't have experience with them.
Buy high ~2000 RPM fans and tune down to 800-1000 rpm.

Motherboards

The most important thing to look for is the VRM quality. There are VRM tier lists for most motherboard chipsets. Here is one for AMD. Aside for this, having multiple PCIe slots is a nicety as well for GPUs and capture cards.
You can tell which boards support 8x/8x by seeing if they two 16x slots with metal shields (for GPUs).
Other boards will only support 4x on the secondary 16x slots.

If you need more than 2 GPUs or need more than 16x in PCIe bandwidth then you will need to go HEDT.
I.e. threadripper.

Power Supplies

Look for something with a 10 year warranty, 80+ Gold or better, and a name brand.
100% Japanese caps is typically marketed on the better PSUs.

Corsair RMx, EVGA Gold, and Seasonic Focus PSUs are good choices.
If you can find a sale, 80+ Platinum power supplies are even better (e.g. Corsair HX or AX).
80+ Titanium is overkill IMO.
Try to aim for a high-wattage PSU (i.e. 600W for any single GPU, 1000W for dual GPU) so that the PSU fan never turns on.

Avoid PSUs worse than 80+ bronze (e.g. 80+ white or no rating). Avoid PSUs with less than 5 year warranties.

SSDs

Try to go NVMe if you can. SATA is okay too if you run out of PCIe lanes though.

Samsung SSDs are the most popular but are very overpriced so I never buy them.
For PCIe 3.0, I typically go for TLC drives such as HP EX920, HP EX950, or Adata SX8200.
These are among the best and typically half the price of the Samsung drives.

If you're on a budget and need a lot of storage, you can go for QLC drives such as Intel 660p.
However, these have worse endurance and performance can suffer for extended writes.

Avoid SSDs without DRAM.
Go Intel Optane only if you know what you're doing (i.e. heavy caching).

Hard Disks

Don't need it. See Data Hoarding for how to setup a linux NAS.

CPUs

As of 2020, Ryzen beats Intel heavily on value and is similar in single-threaded applications.
Go for a cpu with hyperthreading/SMT. It actually makes a difference.