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.
Whoh! Great suggestion - daggerfall unity apparently is free on GoG!
https://www.gog.com/game/daggerfall_unity_gog_cut
Is that the one you’re talking about or is there another version?