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.
For me its Majesty: The Fantasy Kingdom Sim. It is miles ahead of it’s sequel! Both games had a similar take on the heroes reqruited being not directly controllable by the player, but in nr.1 the heroes felt alive (simple and often stupid, but unpredictable often resulting in hilarious or frustrating situations). Nr.2’s heroes felt dull and lifeless to me. Also the artwork and soundtrack (specially hero sound lines) of nr.1 are things i still quote! A bit of an hidden gem and a big time favourite of mine.