I'll never get the love for the irritation that is Nemesis.
-Excellent, unique, and intimidating design
-He's legitimately dangerous and actually difficult to fight. Some of his fights, even on repeated playthroughs, are still a challenge in ways that other RE boss fights don't hold up
-He's the first RE creature to speak and to use firearms, but does such both in such a cool and iconic way, and is STILL better done than any of the talking or gun-toting enemies that turn up in later RE games
-His implementation in the game design was very inventive as he did a lot of shit that wasn't only new for RE games, but for horror/adventure games at the time, such as following Jill through rooms and being a persistent threat that wouldn't "reset" or "despawn" when saving. The normal save room music is even replaced by a suspenseful music track when he's waiting outside.
-He takes on 5+ forms through out the game and he pesters Jill in a huge variety of situations so even though you encounter him frequently, no encounter in any single playthrough is the same
-Even then, Nemesis spawns and encounters will occasionally change on repeated playthroughs
-The game is balanced so that if you put in the resources and skill to defeating him on Hard mode, you're given some REALLY cool and rewarding stuff.
-He's persistent in an otherworldly way. Even when he's a mess of tissue and body parts Frankenstein'd together from a vat of toxic biowaste and acid, he STILL makes a concerted to eliminate Jill.
-His constant presence is overbearing and essential to the extent that it truly does color the entire game and gives RE3 a wholly unique vibe. Other games have tried the "stalker" monster gimmick, like Ustanak in RE6, but the execution isn't nearly as tense, creative, memorable, or most of all, dangerous. Nemesis is a
threat.