Generally speaking, the best way to figure out which mission is best for max XP is to see which one has the most people playing it. It doesn't always work for the first day or so, but by the time you hit Wednesday, enough people have figured it out so that it will always be the most active mission.
Chapter 5 always gives me max XP. For some reason none of the others didn't when I played through them the first time, so I always stick with 5.
Not true (at least if you mean for every episode, not just this one).
Sure, Gagarin was by far the best one for episode two, but Shootout In Valhalla was without question the best for episode three, and that was chapter 4. And Rally Point (chapter 2) is definitely the fastest for episode four.
Two Giants and Refuge seem to be the key maps that are easiest to speed through.
One pattern that still seems to be holding up is that the best chapter for running max XP seems to be the one that has the Red vs Blue easter egg in it.
"Wall of Text'D!" --- oldskoolplayr76 "POwned again." --- blight family
Chapter 5 gives the most, but it's a lot longer than the rest, usually clocking in at around 13-14 minutes for me when playing with randoms. Chapter 2 or 4 usually gives close to the max and are far shorter, so go with those.