It's really a shame on how so many people disregarded this as a mediocre game.
The story in this one is really great and not just the princess is missing go get her. Basically Mario gets a free mansion which turns out to be a ghost mansion, luckily he invites Luigi to meet him there, however Luigi is unprepared to take care of these ghost's so he runs into Professor E. Gad who gives him the Poltergust 3000, to help save Mario from King Boo. Then you start your adventure and it's one of the best yet. The only flaws I could find with this game were there was a lot of backtracking, they did make a warp system, well.. Not a very good one, you can use your Gameboy horror to look into a mirror and it will take you to the front of the mansion, sure this can be nice sometimes, but if they could make a little more warp spaces it would have made the experience, a little more enjoyable. The Poltergust 3000's control weren't that great either, it wasn't terrible but I felt like they could use a target with it or something but it's acceptable for what it is. Another thing that's kind of tedious is you must collect 50 regular boo's some of them are easy and some of them have up to three hundred health and there not hard to catch until they go from room to room, not a huge deal but can be really annoying sometimes. This game gets a lot of slack for being too short too but you know what I would rather play a game that is short but fun through the whole game, then play through a long game that isn't so fun.
The last boss is King Boo in a bowser suit, he is actually quite challenging and you will definitely have to play him a couple times but the overall boss battle is fun, but the boss battle does feels a little bit overpowered, but other than that it makes for a fun final boss battle. The ending is nothing too special but I liked it after you defeat King boo, you take Mario's portrait too Professor E Gad and in a hilarious cut scene Mario comes out with a piece of metal stuck on his head and Luigi is laughing and crying and it's honestly kind of a touching ending, I liked it.
Overall Luigi's Mansion was unfairly criticized when it came out and it didn't deserve it, this game deserves as much glory as some Mario game's did in fact if I'm being honest I liked this one more then Super Mario Sunshine, but you know.., If you like Mario games you will definitely love this game and this may be one of the best games on the GameCube.