[karolkozub]

First of all let me make clear that by "really good" I don't necessarily mean expensive. Instead, the goodness of the pen will be reflected in how pleasant it is to write with it. A good pen feels right in your hand and on paper. Its line thickness matches your writing, and its color matches your taste. Different people will have different preferences and it might take some time to find the right one.

Personally I use "Pilot Super Grip G F" [0] – a 0.7mm black retractable ballpoint pen and I always keep one in my notebook.

[0] https://www.pilotpen.eu/en/super-grip-g-retractable-fine-tip...