When it comes to being attached to certain Pokémon emotionally, I suppose there would be only one for me, which is Metagross.
I've had this Metagross since my first run of Ruby, and while it might not be all that great (its nature and IVs are actually quite bad), I have had the Pokémon with me for over ten years, and it's kind of tough on my end to stop transferring it game-to-game whenever something new is released. If I were to lose it one day, somehow, it'd definitely suck.
Aside from above, I'm also pretty fond of Togekiss. Although I'm not attached to it, per se, I'm still a big fan; its appearance is unique, and on top of this, it's always able to make a great member for almost any team. I've had this Pokémon on my crew for about six game runs in total, and it has never ceased to make good contribution.