So playing the game "Medieval 2 Total War" i noted that some states/kingdoms had only basic gunpowder units (e.g "Hand Gunners" and "Arquebusiers") while other had more professional, elite units e.g "Musketeers". There is also difference between types of artillery different nations could deploy, ranging from basic "bombard" to "Monster bombard" only few nations have at their disposal. So my question is, witch nations were the most proficient in adopting gunpowder in their military tactics?
I am unsure about gunpowder in the form of guns for infantry. I can however say something about artillery. From the later 14th to the later 15th centuries, the dukes of Burgundy (philip the bold, john the fearless, philip the good and charles the bold) were systematically in control of giant "artillery trains". Contemporary sources often revel at the amount of artillery power they could bring to sieges. It is difficult to rank their artillery power in respect to other lordships, especially due to the relatively small size of the Burgundian Netherlands and the duchy of Burgundy. They were however without a doubt among the most powerful artillery forces (if not the most) in medieval Europe. (See Robert Douglas Smith and Kelly DeVries' The Artillery of the Dukes of Burgundy).
Do not overestimate the power of gunpowder artillery however. Although they improved throughout the middle ages, they remained more of a scare tactic rather than the destructive killers you imagine in for example the Napoleonic wars. Although they could kill many enemies, they could just as well explode too soon and kill one's own soldiers.