>>58573364
It's not a clear winner. It's a tie between Unown and Luvdisc.
As for the story: I have been asked it before.
It all started when I got into my first relationship ever. I was a late bloomer and I dated when I was 20 or so. I was deeply in love and as a gesture for our anniversary, my boyfriend of way-back-then gave me a Luvdisc. It was a hacked one with Heart Stamp and the gesture was so cute that I simply fell in love with the Pokémon and all that it represents.
I did a BW run with that very same Luvdisc on Pokémon Black and cherished a lot. Even when things did not turn out well with this boyfriend, I will always appreciate Luvdisc, and how it symbolises my first love and my discovery of relationships and sex. I identify a lot with its lore about romance. And I will always wait for another Luvdisc, even if I no longer play new Pokémon games.
Eventually my liking for Luvdisc turned into fascination as its current circumstances. It is the second weakest fully-evolved Pokémon and a clear example of everything you should not do when designing a Pokémon. Had they made them the Old Rod encounter instead of Magikarp, it would have made sense.
A little bit of "rot for the underdog", another bit of the uniqueness that comes from liking such an unliked Pokémon.