My title of choice would probably be Daggerfall because it’s just so ambitious and truly offers an experience none of the other elder scrolls games offer. That being said it is kinda an example of a sequel eclipsing it’s predecessors as well given it is more or less a straight improvement over Arena in every way.
Honestly, I think that you have a good point. Portal 1 is a tighter experience overall. Once portal 2 got into the goo puzzles I started to lose interest.