Do we need to separate Dragonsong from Heavensward? What about separating 7th Astral Era from ARR?
Anyway, I guess my ranking is:
1. Dragonsong
2. Heavensward
3. Stormblood
4. A Realm Reborn
Dragonsong is a great epilogue and conclusion to Heavensward, touching on aspects that not many writers will touch on. Specifically restructuring of a nation after its fundamental beliefs has been crushed, and diplomacy to repair relationships between a nation that has shut its borders to the rest of the world, and between 2 groups of people who were at war for so long.
Stormblood for me didn't really succeed in characterisation as much as Heavensward did. I do think that Fordola was really well done, but I'm not too impressed with the rest. The opposite end of the spectrum for me was Conrad. Close to zero back story and no connection at all. Most disappointing was Lyse because she was supposed to be the main character of Stormblood, but I didn't find her as well flesh-out as the Heavensward characters. She's not bad, just not great.
Realm Reborn did what it needed to do, but I didn't find it too interesting. Yes it's all there, world-building, light versus dark, evil empire, fighting Primals, and national issues. Not saying that it's bad, just not for me. It really did pick up right before Heavensward though, after the horrendous slog up until then.
I liked Heavensward because I felt that we got to know the characters very well and their personal stories and motivations. My complaint is that we get to see the character's journey, but not enough of what's going on in Ishgard.