My favorite is definitely Raichu. It's very cute, the inspiration is very clever, it's unique, and it's well-deserved.
I actually don't like very many other Alola forms. Marowak is pretty slick and the Sandshrew line is clever, too. Exeggutor is more of a redesign than most, but I'm just sorta neutral on it. But the rest are either just recolors of the original, or extremely close to being so. If I had to pick only one least favorite, it would probably have to be Persian because it had the most potential to be different and cool but completely fell flat (as opposed to, say, Grimer and Muk, who are basically just recolors, but what could you really do to change up their design to begin with?).