Porygon, absolutely. Especially if I can get a buddy to help me evolve it into Porygon2. I remember making a papercraft of sorts of it when I was younger and it's even more special to me now, having a full-odds Shiny one I SR'd years ago in Platinum that turned out to be a square Shiny when I transferred it to SWSH! Would still choose Porygon even if I couldn't evolve it though.
Milcery, Slugma, or Grimer