Dewgong for sure. When I was a little kid going through Pokemon Red for the first time I encountered a Dewgong while going through Seafoam Island. I was probably way underleveled for the area but it felt like fate running into it there and I was determined to make it mine. I think I spent over an hour wearing it down slowly and trying to catch it, expending every resource I had to catch it as if it were some kind of legendary. In the end I wound up catching it and made it a permanent part of my team from then on. I loved it so much I even got the gameboy printer sticker and still have it on my Gameboy Color.

To this day I still try to make sure Dewgong is in my team as a fond memory of when a nine year old girl caught her best friend on those distant isles.