I would say that Popplio is the best starter in Alola because:
1) Water/Fairy is arguably one of the best typings in the game which allows it to resist many different types of attacks and only three weaknesses (Electric, Grass and Poison). Primarina also benefits from having a vast movepool to deal with all sorts of threats including Psychic, Shadow Ball, Energy Ball.
2) Primarina's stats consist of 80 in Health, 74 in Attack, 126 in Special Attack, 116 in Special Defense and 60 in Speed, which makes it specially bulky.
3) Primarina has the advantage against the following trials: Raticate, Marowak, Kommo-o, as well as the Trial Captains Kiawe and all the Kahunas (Halu, Olivia, Nanu and Hapu).