Depends on how many you need and how much time you have.
If I need more than 2, taking endless Void to rotation C (your choice of mission) is generally my preferred way to go. It's RNG but more likely than not to pick up enough. 15-20 minutes of play, easier if squad does not bail on you early.
If I need 1 or 2, it's about speed. Void Exterminate or Capture and break containers on the way. Keep an eye out for the special Argon drop. On average get 2 every 3 missions, sometimes luck out and get 3 in one mission. Takes about 2 minutes to run it, so less than 10 minutes to be reasonably guaranteed to have all I need.
I always do the T1 branch if I can. More people PUG it, and I generally don't want a challenge if all I am after is a measly Argon crystal.
T1 is what most players call the "easy" branch of the void. There are 4 separate branches and their difficulty levels all correspond directly to what the T1-T4 difficulties used to be (together with the T4 damage bonus). It's better than calling it "Phobos branch" or "Europa branch" because most players know what you're talking about.