They're all simple and adorable. The popularity race is so far pretty similar to when 5th Gen's starters were first revealed - Grass is currently the extremely popular one, Fire is second popular while Water is getting hated for being a derp.
I personally do love Popplio the best, because it reminds me so much of Oshawott with the nose and the fact they started out as the least favoured, and we know where Oshawott went with the awesome evolutions so I have hope Popplio will evolve into something cool and win fans from the other two.