trading and csgo keys

Any. They are considered a currency and they all go for the same price (give or take a few cents). It does not matter which key it is since only a fraction of the buyers actually end up using them to open a case.

They're mostly looking for the new ones, but you should always prefer the ones that currently have a nice price on the steam community market AND they are in huge quantities in the steam community market. So, for example, don't choose the esports key, because only 184 are being sold on the steam community market. Also, always avoid the common csgo case key (also called vanilla), because nobody wants that.