Yep, DS2 is far and away the worst game in the series.
You can preach all day about how great the build variety and PvP systems are, but if the moment to moment gameplay is so much worse than the others that's entirely irrelevant to me.
The core gameplay systems are way too sluggish, attacking, rolling, healing, it all feels so much worse than it's brethren. Even with level ups you're forced to put points into adaptability just to make the game feel better to play.
Then there's the weirdly balanced healing. They limited your estus (especially early on), but then added in a farmable healing item anyway which didn't make a whole lot of sense.
Level design is trash. Empty rooms, empty corridors, packs of enemies, it all feels like levels rather than parts of a world (your example of Anor Londo vs Drangleic Castle is spot on). I do appreciate the variety in areas and visuals, but it's a hollow victory with the above issues.
Then the boss fights. My lord. You have way too many big knights, you have a big rat that moves eerily similar to Sif, and a literal pack of trash mob rats. The few inspired designs (like the frog thing) are so underwhelming as well that the initial awe disappears in about 10 seconds.
Those three aspects are the bread and butter to me, and the reason I loved DS3 so much. Playing that really highlighted DS2's missteps even more. The core gameplay was silky smooth, dodging felt great, attacking felt great (I used a katana + pyro build), healing felt great.
The levels were great, large areas, multiple paths, cohesive world.
But the boss fights, lawwwd. So many fantastic designs with great moves to match them.
Aldrich is one of the most stunning designs in the series, with a cool battle to match it. The humanoid battles are great. Abyss Watchers, Pontiff, Dancer, Dragonslayer Armor, just fantastic all around.