Here is what I think.
When I first saw Litten, I thought "Is this really a pokemon?" For some reason, Litten doesn't look like a starter to me at least. I mean look at Fennekin and Chimchar, Litten has a weird design and body shape imo. Will probably and hopefully turn into a Fire/Dark Tiger.
Popplio was just weird to me at the beginning, but now that I've seen the starters for so long, I find it cute and rather amazing. The water bubbles thing is actually fun. Logically, I would say it will become Water/ Psychic because of its ability to control bubbles and all.
Rowlet, although it lacks the starter feel too, is imo the most interesting starter for me. Thank Nintendo there are 3 starter birds of different types now. Wish fulfilled! Anyways, the head spinning 180 degrees thing and a new move for it! Its so cool! Grass/Flying makes it the second starter to get a dual typing in its first stage but its fine with me. Also, reading the full descriptions given of Rowlet and realizing that they have mentioned "It has no problem in darkness", my guess is that it could be a Grass/ Fairy or they might just make it a Grass/ Flying when it evolves.
And if I had to predict which starter Ash will get in anime, I would say Rowlet because of its ability to spin the head around to take commands from its trainer. Litten will probably be chosen by the rival.