Tough but what a fun question!! And I really can't choose between the water types, so I'm just gonna list these idc.
Cloyster, Gyarados, Quagsire, Omanyte

YES so I like Cloyster because I love shells basically, so same reason for Omastar (and he has tentacles but w.e), Gyarados was always on my list of fav Pokémon in general and Quagsire because I CAN RELATE SO MUCH TO HIM /ability @ unaware and I love his typing and design etc.
Vileplume
I love the whole Vileplume line really. Poison is one of my favourite types and I LOVE the design and the concept too, spores, stuff like that. I AM IN LOOOVE with Gloom too because yeah well same like Quagsire it reminds me of myself and all around they're just so.. sort of mischievous looking, they just look evil-natured lmao and I love it.
Arcanine

Apart from there not really being many fire types besides starters, Arcanine is a cutie and his concept of a chinese legendary dog being is really cool and I love his design anyway.
SORRY I WILL GIVE REASONS I FORGOT ^