Cloyster is my favorite Pokémon. He was my favorite before I even started playing the main Pokémon games, mostly because of the game Pokémon Puzzle League for the N64. If you've never played that, Google it. Anyway, one of the playable characters is Lorelei, and one of her Pokémon was Cloyster. That's when I heard his magnificent call (Clllllloyster!) and his design was awesome.
Even after I started playing more of the Pokémon games, Cloyster remained my favorite. As it turns out, he is amazing in battle. With access to Skill Link, Icicle Spear, Shell Smash, and a crazy high Defense stat, he can sweep Pokémon teams. When I played my first Pokémon game, which was Pokémon Y, I went out of my way to find a Shellder and a Water Stone for the sole purpose of having Cloyster. I did the same when Pokemon Sun and Moon came out. When I Played pokemon Blue on Virtual Console, I did the same (And his Sprite in that game looked funny because his shell opens horizontally, which it's not supposed to) And you better believe that when Pokémon Gold and Silver come to Virtual Console, I will find my beloved Cloyster.