[2019] Which skills are 'Must Haves' for general PvE & Endgame?

Ive been solo so far, up to the last stronghold, and ive been using anything i want because its a pretty easy game for the most part if you go after the stuff geared for it, so seeker mine and bullet drone. If i have a tough end guy ill use the drone to eat up armor. Seeker mine just because its fun. when it gets more difficult i might have to really think more about it.