I'd thought of most of these, but 2. is actually the most compelling. I totally forgot physics applies to these creatures - the spear didn't kill Viserion, it probably only severely wounded him, it's the impact that did it. The Night King, calculating this, specifically went for a flying one, to use gravity as a partner in crime. In reality, I still think it's illogical/poorly written, because I feel that a spear in the lungs/heart would have killed Drogon, too, but it's the most compelling reason yet, especially in combination with some of the others. Bravo. Upvoted and thumbs up.

I don't think the death on impact theory is really plausible. His guts literally spilled out. He wasn't going to survive that wound, no matter what.

@DavidGrinberg: The Night's King also could not conclusively have known that there would be a guaranteed death on impact. This question/answer focuses on the Night's King's reasoning for targeting Viserion, not what actually happened when he threw the spear. And as per GhotiandChips' comment, Viserion received the double whammy of spear+gravity. Even if not directly fatal, it will still hurt more than only the spear (as would have been the case for Drogon).

@GhotiandChips: because I feel that a spear in the lungs/heart would have killed Drogon, too I only edited my answer just now, but your comment here touches on it. From what I saw, Viserion was hit under the wing; which is likely less armored (due to needing free range of movement). Going by the picture in your answer, much of what the Night's King could see of Drogon seems to be armored or covered by his wing (he kept his wing down because he was leaning on it, not flying), thus making less weak spots available to be hit, compared to the overhead dragons. IF it was intentionally aimed.

It's a story. It wouldn't make narrative sense to kill the representative dragon first This is GoT. We used to sit at the edge of our seats to see our favorite character make it out alive, because there was a damn good chance George R. R. Martin would take them out. A good rule in general, but it doesn't / should't apply to GoT, though it seems to nowadays. +1 on the rest

@rath: GoT still doesn't kill off its true narrative characters. It simply created red herrings, people that we assumed to be narratively important (Ned Stark being the first and clearest example). In-universe, there is little reason to choose which dragon dies. While Drogon is bigger, all three dragons have the same anti-walker traits. But Drogon's death would require another dragon to be introduced as the representative of the dragons, which is narratively pointless. Similarly, Thormund will likely live for as long as the wildlings need representation (unless his death is important).

Another point: If Drogon were hit first the other dragons would stay to protect / extract their Mother, who is now unhorsed and surrounded by dead things with pointy weapons. It would make perfect sense for Drogon to go first, but it seems Gorkem's comment is probably the right answer : he aimed Drogon first but seeing the other dragon targeting him he had to change target

If I may add to your speculation, it seems like a dragon's wing in flight would be a lot larger and easier to hit target than the dragon himself. It would be sort of like attacking a charging knight by shooting his horse.

One final speculative out of universe explanation: there's a Targaryen who may end up riding a dragon and it seems fitting for him to ride the one named after his father.

@T.E.D.: It wouldn't be easy to hit, the wing is the thing that flaps around the most. If anything, it's the hardest to hit (regarding its movement, not its size). Even then, a spear would only poke a small hole in the wing. That's not guaranteed to prevent the dragon from flying. And even if it does, the wing will still sufficiently work for the dragon to land safely (enough), and continue its onslaught. I'm not quite sure why you'd use the analogy of shooting a knight's horse, rather than just shooting a knight in the legs. And a limping knight may be slowed, but will still try to kill you.

@JaredGoguen: Fully on board with that theory. Jon has been seen staring at Rhaegal (unaware of the connection) and it makes perfect sense. Not only is Rhaegal relevant to Jon, Viserion is also named after the least likable character of the three. That made him the prime target for an early death, compared to the other two (greedy Vyserys vs indomitable Drogo and widely loved Rhaegar)

"We don't know much about the ice spear and whether it has any magical qualities that help with killing dragons." It seemed pretty obvious to me that the ice spear was magical, considering it didn't shatter on impact against a dragon's hide (ice is very brittle, especially in spindle form like a spear), and considering Viserion literally started deteriorating in mid-flight.

@TylerH: Your assumptions are not (proven to be) correct. The ice spear was thrown with a massive amount of force. Given enough force, and a spear tip that is sharp enough, an ice spear will penetrate materials that are softer. My answer also explores that Viserion was hit in a weak spot that seemed to not have armor (due to being at the base of the wing, a mobile limb). Knights' armor has historically been weak at the joints for the same reason: the ease of moving your limbs. Also, if the spear does shatter, its traveling speed simply turns it into high velocity shrapnel. Still dangerous.

@TylerH: Even if the strength of the ice spear is affected by some magical process, that does not mean that the ice spear's magic specifically affects the dragon on a magical level. Upon rereading, I noticed that your comment doesn't argue about the ice spear having a magical effect on Viserion. And that was my point in the answer: there is no need for magic in order to pierce a dragon's skin, as evidenced by Qyburn's ballista.

As an (ex) turkey hunter, I can say that the money spot is under the wing. The heart is there, and the main protection (at least topically) is the wing itself. The wing can easily stop some birdshot from penetrating the heart. So yeah... for whatever that's worth...

Possibly worth adding, a dragon dead on top of the rock would potentially have been burned by the dragon fire of the other two, denying the Night King a resurrected dragon. By targeting one that was flying over very thin ice, the Night King knew that the corpse would be protected.