I thought all three were pretty cute, but I ended up picking popplio because I liked her final evolution more, the mermaid seal.
At first I wasn't able to do much with this pokemon, but when she got to her final stage with the fairy typing I found my Ariel to be a pretty strong special tank. Nothing was left standing after an Oceanic Operetta from her. I'm glad I picked her and remained patient.
Its going to be hard putting her aside for new playthoughs, as much as I would like to give cute rowlet a try, I like my water starter a lot, and for nostalgia sake I will probably always pick popplio like I do with Snivy.