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 or VA 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.
Note that some lower-end GPUS and adapters won't be able to output 4k60.

Peripherals

Main article: 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 and NH-D15 are excellent for ~$75. They will last 10+ years.
Online reviews have also praised the NH-U14s and Scythe Fuma 2 for ~$60. However, I haven't used these.

If you're on a budget, a Hyper 212 or GAMMAXX 400 can be had for ~$25.

Fans

Prefer Noctua. Their fans will last 10+ years.

Avoid all sleeve bearing fans.
Cheap sleeve bearing fans make ticking sounds with DC fan speed control.
Try to find PWM fans if you can but most motherboards will support DC control.

Arctic fans are supposedly good budget fans but I don't have experience with them.
You can buy ~2000 RPM fans and tune down to 800-1000 rpm in the UEFI/BIOS.

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 nice for multiple GPUs or capture cards.
Both Ryzen and Intel consumer CPUs only have 16 lanes dedicated for PCIe. Other lanes are shared with the chipset.
Often, 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 going through the chipset.
Some workstation motherboards will have PCH switches which allow for connecting more devices sharing the total bandwidth.

If you need more than 2 GPUs or need more than 16x in parallel PCIe bandwidth then you will need to go HEDT (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).
Most expensive PSUs will keep the fan off when load is under 40% so try to aim for a high-wattage PSU. If in doubt, check the LTT Fourm PSU Tier List

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 more expensive than others so I never buy them.
For PCIe 3.0, I typically go for 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 large extended writes.

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

Hard Disks

Avoid for PC builds. It's not worth the weight and noise. See Data Hoarding for how to setup a linux NAS.

CPUs

As of 2020, Ryzen 3000 CPUs beat Intel 10th-gen CPUs heavily on value ($/thread), perform similarly in single-threaded applications, and offer PCIe 4.0.

RAM

RAM is one of the safest things you can buy used.
I always test RAM, both new and used, upon receiving it with Passmark's Memtest86.
Failing memtest86 means either the ram is bad, your OC is bad, or your CPU's memory controller isn't good enough for the OC.
Sometimes, an XMP OC can be good with a small amount of ram but will fail with more ram due to a week memory controller (e.g. on Ryzen 1000 CPUs).

For XMP overclocked ram, it's best not to mix ram models.
For JEDEC stock ram, it's usually okay to mix models as long as both have the same specs.

In general, you want the highest MHz and the smallest CAS latency.
I recommend sticking with reasonable speeds to avoid issues such as random BSODs or random restarts.
For Ryzen 3000/5000 and Intel 8th gen, only up to DDR4-3200 is officially supported.
Intel CPUs tend to support faster RAM speeds as Ryzen CPUs' Infinity Fabric clock (FCLK) have their memory frequency directly tied to memory frequency.