I've always liked the idea that the knights come back before Arthur which i hardly ever seen done (probably because theres little angst and lots of happiness/humor) or the idea of Lancelot coming back first (by like centuries) which ive never seen done (which would be happiness for Merlin but lots of angst for Arthur) but now im really liking the idea of everyone coming back in the order that they died: Lancelot, Elyan, Gwaine & Arthur, Leon & Percival
So Lancelot pops up first. He died 4 years before anyone else so either he shows up 4 years before everyone or its like multiplied bc immortality and he shows up 40 years early. He gets used to the new world and Merlin's sad sad eyes and after the hurdle of his first year they start actually having fun again and living. Maybe they know why he's there maybe they dont
Next is Elyan, who died about half a year before Gwaine and Arthur. He pops up and it throws Merlin and Lancelot for a loop because either they werent expecting him or were expecting him at a different time. He's super confused. With both Merlin and Lancelot he adjusts a little quicker than Lancelot did but its a much harder adjustment (Lancelot has come back from the dead before with a time jump. His difficulties were with physical things, Elyan's is with emotional things)
Gwaine and Arthur died on the same day, so they return at the same time. They both immediately get into trouble but their panic is much lesser than Lancelot's or Elyan's was because they're together. They come home to a blaring TV, plastic snack trash everywhere, flashing colored lights, and Merlin's pet macaw(s) (they were having a tv marathon when they left to pick up Gwaine and Arthur). Its overwhelming. Also the bird(s) are scary. They do not adjust well. Gwaine punches the microwave and Arthur pretends to understand things he doesnt.
Now we dont know when Leon or Percival died, but they'd come next. Gwen too if you want to include her. We also dont know how they died, maybe they're the only two who come back as old men, which shocks everyone else (but Merlin. He was kinda there for that) because of change of appearance, change of abilities (decreased), and change of personality (you change as you get older).