There is not much to debate. If you don't have IoD and you don't plan to buy it with cash for the other shinies like tomes or hirelings, then IoD wins, because it is significantly more expensive than the other expansions.
If you already have IoD or want its accessory shinies, then Feywild, because it is also significantly more expensive than Saltmarsh, which is a mini-expansion.
Saltmarsh has against it:
-Less quests - only 10 quests!
-Lower price - easier to buy
-No raid
Saltmarsh isn't even the best place to farm sentient xp. There was a time when sentient stones had very good drop here, but now it has dropped to ridiculously low levels. The only thing that Saltmarsh has in its favor vs the others is the spark of logic, but I would buy this mini-xpack with points, since it costs the least of all. Saltmarsh is a mini-xpack, the other two are full xpacks.
The fun of quests is subjective. I like the quests in the three packs. In this sense, the quests I like the least are in Vecna, which is not included in the giveaway.