Cinnabar Island, mostly for the Pokemon Mansion, which I always looked forward to exploring. I also liked Mt. Moon, Mt. Ember, and the Seafoam Islands for similar reasons (they were fun to explore), as well as for their interesting scenery and/or folklore.
Seafoam Islands, because they are actually fun to explore and have puzzles, and because Articuno is my favourite Kanto legendary. In the remakes, the cave also looks pretty. I also like Pokemon Tower and Cerulean Cave for their creepy atmosphere, and the latter is great for grinding (shame they dumbed it down in Generation IV).
Cinnabar Island's Mansion because of the diary entries about Mew, especially since it was almost impossible to legit catch Mew at that time. Also liked Unknown Dungeon and the Safari Zone because of the challenges in catching Mewtwo and getting to the end of the Safari Zone.
I have to say Viridian Forest is the one I enjoy the most. It's peaceful and it just feels perfect as a place to start the journey.
Honourable mention - if it counts - goes to S.S. Anne for setting up a rather pleasant training part of the game.