Before they gave us a difficulty selection for solo duties I would've said Stormblood because some of the solo duties used to give some people pause and you had The Royal Menagerie which somehow seemed to have gotten progressively harder to clear the later in the expansion we were, analog to The Dancing Plague now.
Now that we have the option to play MSQ solo duties at a lower difficulty if need be I find it hard to single out a particular jump in MSQ difficulty from the previous expansion, partly because it's been way too long since I've leveled through the content to have a clear recollection of its perceived difficulty let alone how difficult the content is for players going through it now.
My personal impression of, say, Titan will always be colored by how things used to be years ago (when the server locations were different, had like 2-3 seconds of latency by default and made the fight harder than it should have been) but things have changed since then and players doing the fight now will naturally have a different impression (and then there's power creep shifting subjective difficulty further).

If I had to pick one now I'd probably say Shadowbringers because it's the newest and as such is the least affected by system changes and power creep.
All of this, though, is looking at the relative difficulty of the expansion's MSQ content as a whole.
If it's just the initial difficulty at the start of an expansion we're looking at I feel like all of them start off gentle enough, especially so if you go in with augmented tomestone gear from the previous expansion.