Monitors are one of the most difficult components to choose, as you have to find a good compromise between its pros and cons. The first general advice is to ignore all the manufacturers' specifications because very often do not correspond to reality. Most of the time the actual values of contrast, brightness, response time, viewing angles, etc. do not come anywhere close to those indicated. The only way to know the true quality of a monitor is to rely on the reviews. The important information to take into account is:
Resolution: 1080p is the standard now. Only monitors smaller than 22'' have a lower resolution. Up to 27'' is an acceptable size for a monitor (and included it has a full HD resolution), while more than that may be poorly defined. However, with increasing resolution, it will also increase the hardware requirements for games. And take into consideration that it is not recommended to buy a monitor with a 4K resolution, if the computer is not able to handle it properly.
Panel technology: Until a few years ago, almost all the panels were of the TN type. These panels have the advantage of being very cheap and have a good responsiveness, but the quality of the colors, contrast, and viewing angles are left to be desired: for these reasons they are destined to become obsolete. Lately the price of IPS panels has gone down dramatically, this has made them very competitive and a popular choice. The advantages of IPS panels are superior viewing angles and color fidelity, but they are also generally less responsive compared to TN panels in the budget segment. VA panels have improved a lot in the last years and now perform almost like IPS panels, with 2-3ms response times and up to 240Hz refresh rates. They exceed standard RGB and often reach the much richer and wider Adobe RGB color gamut, also having 178/178 wide viewing angles. Because VA panels offer impressive contrast ratios (3000:1 and more are common), they’re great for HDR content. New OLED panels offer superior image quality, more efficient power usage, and a more compact design compared to traditional LCD panels.
Refresh rate: if you are a gamer, this should be your primary concern when picking a monitor. High refresh rates like 144Hz or 240Hz ensure incredible fluidity in games by decreasing input latency. New monitors can even support 360Hz refresh rate.
The perfect monitor does not exist. So what should you purchase? If you have a limited budget, you do not have many choices. You will have to necessarily settle for a 60Hz TN.
For those who want to enjoy the most out of the video games, especially the fast-paced ones like FPS or sports games, you should opt for a 144Hz or higher monitor.