That's only simple if you have a power meter. A much simpler version is "someone who does better going uphill than on flat terrain." And the OP seems to follow that definition, certainly as far as the group that he rides with is concerned.

Even defining it in terms of W/kg is a bit too simplistic, different riders use their power ratio differently, or can sustain it for different lengths of time. You get small sprinters like Caleb Ewan or Oscar Freire, and you get big climbers like Indurain or Dumoulin (Tom, not Sam), and you get puncheurs who are good on short climbs and can win in the Ardennes (Kwiatkowski, Alaphilippe, Rodriguez) who are not the same as the guys who can sustain it over long Alpine passes (Contador, Majka). There are guys at the top level who have better W/kg than others, but might be spent by the time they get to that last climb of the day and can't hang on.