Made a heavy bowgun set, but what do you guys consider a most have skill for bowgunner?
And what do you consider the best 1 socket decoration that gives +2 skill points? (I currently like using lasting power for extended might seed/pill duration)
Depends what bowgun you're using. For LBG, rapid fire is obviously pretty good. Then status for a status gun (like kettleblower), bomb boost for crag guns (like rusted/ancient or los LBG), and element boost for element guns (fire for los, thunder for lagi, etc). Pierce up for pierce HBGs (blos or narga). I mean, it's just stating the obvious, but each gun has a pretty strong identity, and you're best served by sticking with it and having the corresponding kills.
I also like attack up. I don't really care for load up or quick load. For the first, I don't feel much of a difference when I'm sticking to rapid fire or siege fire. For the second, it could mainly be nice for HBGs, as they're slower to load, but siege mode negates the need to actually load. Load up could be helpful if there's one specific ammo type you rly like, but it's not a normal strength of the gun you're using (and it only loads a few), but then you're better served by crafting the right gun for that ammo. It could still be good for some of the more exotic ammo type, maybe slime or whatever, but it's not worth it to me in general.
Welcome here, enjoy your stay (and the game)!