The best monitor for your setup depends on a range of factors, including the purpose of your PC and how powerful the PC is, so we'll look at this one monitor purpose at a time:
Monitors for gaming are another proposal altogether. 4K gaming may sound like a fantastic experience, but if your PC can't drive your games at 4K effectively then you're not going to get far. Similarly, many monitors now offer refresh rates of 144Hz and above - and while not reaching this won't actually make your gameplay experience any worse, if your gaming PC is only playing the game at 50FPS, there's not much value in spending more on that 144Hz monitor.
That said, with a PC that can push a gaming monitor to its limit, the difference is astounding. Going up a resolution is generally a one-way trip for good reason - making the leap from 1080p to 1440p, or to 4K, can completely change your experience and if your rig and budget support it, go for it. The same goes for refresh rates, though to a lesser extent - if you play high-paced competitive games then a 144Hz monitor upwards can make a world of difference.
For Office PC screens you have to consider screen size, resolution and if relevant, VESA capability. Multiple displays is a must for multitasking, so any office PC monitor setup should be along the lines of two 24-inch 1080p displays. This offers a good screen-size, industry standard screen-resolution and your other main consideration should be a quality, reliable brand to ensure you won't be replacing them short-term.
VESA is the industry standard mounting system, so if you're planning to mount the monitors on a desk or wall bracket, check for this - it's the 4-screw square pattern that's visible on the rear of most monitors.
A note on connecting your monitors - if you're using multiple displays you'll need to ensure your system has sufficient ports available to support the connections your chosen monitors use. All modern monitors will support HDMI, but if you only have a single HDMI port available on your PC, then you'll need to be mindful of this and check that the other ports available on both your system and second monitor are compatible.