Can I have one three from each gen? One poké alone is too hard to choose... too damn hard!
I:
Dragonair: this thing's just as beautiful as it can get.
Gengar: the original ghost, the phantom, the shadow, the unknown hiding in the darkness... mischievous and eeriee design, I love it.
Clefable: the original pink cutey, and also the counterpart of Gengar. What else do I need? Oh that's right, Magic Guard!
((Honorable mention of Vileplume's line, Lapras and Vaporeon, my fav eeveelution but not my fav mon from gen I)
II:
Misdreavus: cutest non-evolved ghost hands down. Also it looks to me like a bodyless head, which is pretty scary.
Azumarill: whenever I think of this thing, I can't avoid to relate it with Easter eggs and the Easter bunny. Plus is so adorable.
Jumpluff: no matter how, always I battle this thing, it outspeeds my entire party and puts everyone to the likes of sleep or poison. It makes me so mad that I can't help but love it XD
((Honorable mention of Corsola, Skarmory and Girafarig))
III:
Sableye: is so mean looking, it seems as if it were to eat your face out at any moment...any moment now....
Mawile: I love the base of the design, and its deceiving and treacherous nature.
Gardevoir: the best psychic type around IMO. So elegant, and I love how everyone always seems to think it can just be female XD
((Honorable mention of Roselia's line, Deoxys and Kecleon))
IV:
Giratina: let me explain this to you: Is a ghost dragon.GHOST.DRAGON. If you can't get the badassery of this one...
Froslass: a beautiful looking ghost based on an interesting legend. I always thought her head looked like a Snorunt's dead body...
Dusknoir: this thing always reminds me of Oogie Boogie (dunno...) and since that's one of my fav villains ever, Dusknoir gets in this list.
((Honorable mention of Weavile, Togekiss and Gastrodon))
V:
Audino: I'm all for the cute stuff, and Audino might be "useless" (*EXP* tho) but is the cutest pink poké you'll get that is NOT a fairy.
Leavanny: she might be weak to almost everything, but I've seen no other bug type as beautiful as this one.
Reuniclus: is the CELL pokemon. Plus it's a rabbit. I love biology, and cells, and rabbits. Did I mention that it's the CELL pokemon?
((Honorable mention of Frillish's line, Serperior's line and Chandelure's line))
IV:
Gourgeist: cutest fully-evolved ghost hands down. The hair-arms, the psycho instincts (it drowns you for fun!), the tiny fang that makes me think of Kyoko from madoka.... dunno what else to say, some might preffer the badassery of Trevenant but I'll go with the dreadish cuteness of the halloween poké ^-^
Malamar: I love its evolutionary line, it goes from the cutest thing ever to the most malicious, kraken-like creature around. Plus contrary <3
Goodra/Delphox: yes, this is a tie. I cannot choose. Delphox was my first starter ever, but Goodra is the cutest fully-evolved dragon I've seen.
((Honorable mentions of Florges' line, Espurr's line and Sylveon))
EDIT: Reading all the things people've writen, I consider I missed a lot of mons that are also favs of mine, so 18 pokes will be making this list now.