The first game where Luigi gets the spotlight and builds on his personality, I love it!
It's an interesting concept to set a whole game inside the one mansion, but it works quite well. Making your way through on your first playthrough is so fun, and sucking up ghosts and collecting treasure feels amazing. The game... is actually pretty creepy for a Mario game. The presentation and sound design make you feel unsafe no matter where you are, and I like the detail of Luigi humming to fill the silence as well as him and the music and sounding different depending on how much health you have! The GameCube's graphics honestly add to the spooky experience as well. Despite that, the graphics are incredibly good for its time.
I really love the tidbits of story here and there too, with the information on the ghosts that had lived in the mansion along with Mario's story. I think the ending could have been so much more, though. The final boss is so cool and unique for the style of gameplay this game has, but I felt like the game ended too quickly after that for me to process it was over!
The Hidden Mansion is a great post-game mode though and I look forward to working towards beating it and getting the best rank I can. Mirroring the mansion is a surprisingly effective way of adding replayability and way better than nothing, and you get to really test your ghost-fighting skills as the game throws you tons of ghosts at once. It's a very silly challenge and I love it so far.