My absolute favorite Pokemon is Marshadow
My favorite type has always been Ghost. When I had Blue as a young kid, I originally picked Haunter as my favorite Ghost type... only to later find out that it evolved into Gengar, who my brother had claimed as one of his favorites. Since it is impossible for siblings to have overlapping favorites, Gengar was blocked from becoming my new favorite, so I was determined to get a new favorite ghost in Gold... Only for it to turn out to be the case that the only new ghost was Misdreavus, who was locked away in the post-postgame. I was still determined to make Misdreavus my favorite for a while, but I was always a bit disappointed that competitively it remained outclassed by Gengar. It had its niche with Perish song, sure, but continued to live in Gengar's shadow.
Ever since, every generation I would hope for a new ghost type to become my favorite.
Gen 3? Banette was ok but somehow didn't rise to the ranks of favorite.
Gen 4? I had high hopes for Mismagius but it remained outclassed by Gengar, and after waiting and hoping for a Misdreavus evolution it just fell short---though I think the main issue was that I didn't even get to use one on my in-game team due to the fact that there was only one dusk stone in the game, so it was too important to use on a random in-game Misdreavus, but I don't think I ever finished my optimized team. Giratina was probably my favorite ghost at this point, but didn't rise to overall favorite. I think I probably liked Darkrai quite a lot at this point---it was what I wished Misdreavus could have evolved into, and I was annoyed it wasn't a ghost type. But, at the same time, this was back in the days of it being next to impossible to get mythicals, so I never even had a Darkrai back then.
In Gen 5? Chandelure had a good shot at becoming a favorite, especially with the tantalizing prospect of it getting shadow tag, but of course that never released and was replaced with infiltrator next gen, and within gen 5 its meh speed once again meant that Gengar outshadowed it.
Gen 6 finally delivered an amazing ghost type in Aegislash, and it became my favorite for a while. Mega Gengar was also amazing, and brought Gengar back into the favorites fold. At this point, the only things these mons were missing was shoulder-mon status. Pikachu's iconic hanging out outside of a pokeball felt like a nice feature for a favorite to have. Hoopa was the right size for a shoulder mon, but I actually didn't really like its design very much -- and then Hoopa unbound wasn't ghost anyways (and was a lot uglier).
Then, gen 7 rolled around. During the prerelease period Mimikyu was revealed, and it was a strong contender for a possible new favorite: great typing, neat ability, shoulder-mon. I didn't quite like the lore of an entire species of Pokemon cosplaying as Pikachu, but I could maybe accept it through a head-cannon that the species could make spider-silk and just made their outfits out of that. Decidueye also might have been a contender for favorite once it was revealed that it was a ghost-type, but then, the datamine of the demo happened, and images of all the Pokemon were revealed. I remember seeing Marshadow and thinking "that mon looks so cool, but GF will probably pull another Darkrai on me and it won't be ghost type." And then, once the game was finally datamined, not only was Marshadow a ghost, but it had one of the best offensive type combinations in Ghost/Fighting. And it had an absolutely bonkers move in Spectral Thief, plus a signature Z-move to boot. That sealed it. This was my new favorite Pokemon, and it has been ever since. I got two copies during the distribution, and saved one to use during my playthrough of Ultra Sun, using a mono-ghost team. This remains my favorite playthrough of the entire series.
In gen 8, once Dragapult was leaked and revealed to be ghost/dragon, it shot up to one of my top favorites, surpassing Aegislash but not Marshadow. Hisuian Zoroark (and Zorua) also blew by Aegislash to the top of the favorites list, just behind Dragapult.
Finally, with gen 9 came the release of Flutter Mane, which was everything I had hoped a Misdreavus evolution/form could be (except for its stupid snaggle tooth and name). It's not enough to push passed Marshadow, but Flutter Mane is now my #2 favorite ghost, ahead of Dragapult.
At this point, I doubt Marshadow can be dethroned. It is everything I wanted in a favorite Pokemon. I've bought basically all of the Marshadow merch I could find at this point: the plush, the TGC box with the Marshadow figure, the version of that same TGC box but that came with a jumbo card instead, the Marshadow TCG tin, a game-stop exclusive Marshadow figurine, the Marshadow pins, socks, and sweater from the Pokemon Center store, most of the Marshadow TCG cards (except the Sylveon card with Marshadow on it that goes for like $100+ on Ebay ;_;), Japan-exclusive(?) TCG card sleaves... If there is any other official merch out there, I am unaware of it (and please let me know if it exists...!)