IDK.
Plaguelands is better than Dreadnaught.
WotM is better than KF.
Forge is better than CoO.
LL system is better than in TTK (not really fair, but still)
TTK did introduce new subclasses and many changes, including a reset of all weapons and armor.
Plaguelands was the cosmodrome with snow.
And TTK had a much better story even if you only played it once.
And AF had the benefit of coming after CoO.
Once you cracked the initial secrets in the Dreadnaught, it was a pretty boring place to play. Plauguelands is at least consistently interesting and engaging, even if they did re-use a region for expediency. It's just more fun to play there.
TTK had better deep lore, but the main story wasn't very good. Half the campaign was devoted to trying to open a door. This was masked by great character and world building, but there's really not much happening other than Oryx showing up and we try to get to the Dreadnaught and open a door to find and kill* him. RoI wasn't a great story, but I thought it was more engaging and the reveal of what happened to the other Iron Lords was wonderful moment, and led to a good fight. The story tied together in a way TTK didn't.
It's not really apples to apples given everything else TTK brought to the table, but I enjoyed the story missions in RoI more (and played them more), the location and the story. RoI had the better raid, but fell short in the strikes department. A wash overall, but the base location/missions, I thought RoI was the best so far.